diff --git a/backup/util/helper/backup_array_iterator.class.php b/backup/util/helper/backup_array_iterator.class.php
index 1fce832ae77..ae0b8a4f521 100644
--- a/backup/util/helper/backup_array_iterator.class.php
+++ b/backup/util/helper/backup_array_iterator.class.php
@@ -40,23 +40,25 @@ class backup_array_iterator implements iterator {
         $this->arr = $arr;
     }
 
-    public function rewind() {
-        return reset($this->arr);
+    public function rewind(): void {
+        reset($this->arr);
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return current($this->arr);
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         return key($this->arr);
     }
 
-    public function next() {
-        return next($this->arr);
+    public function next(): void {
+        next($this->arr);
     }
 
-    public function valid() {
+    public function valid(): bool {
         return key($this->arr) !== null;
     }
 
diff --git a/backup/util/helper/backup_null_iterator.class.php b/backup/util/helper/backup_null_iterator.class.php
index 80eb5301a56..9d317e00d91 100644
--- a/backup/util/helper/backup_null_iterator.class.php
+++ b/backup/util/helper/backup_null_iterator.class.php
@@ -34,19 +34,21 @@
  */
 class backup_null_iterator implements iterator {
 
-    public function rewind() {
+    public function rewind(): void {
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
     }
 
-    public function next() {
+    public function next(): void {
     }
 
-    public function valid() {
+    public function valid(): bool {
         return false;
     }
 
diff --git a/calendar/classes/local/event/entities/repeat_event_collection.php b/calendar/classes/local/event/entities/repeat_event_collection.php
index e25a3dc2b8c..0188eac3f95 100644
--- a/calendar/classes/local/event/entities/repeat_event_collection.php
+++ b/calendar/classes/local/event/entities/repeat_event_collection.php
@@ -99,7 +99,7 @@ class repeat_event_collection implements event_collection_interface {
         );
     }
 
-    public function getIterator() {
+    public function getIterator(): \Traversable {
         $parentrecord = $this->get_parent_record();
         foreach ($this->load_event_records() as $eventrecords) {
             foreach ($eventrecords as $eventrecord) {
diff --git a/calendar/tests/action_event_test.php b/calendar/tests/action_event_test.php
index c17a524375f..55881793dba 100644
--- a/calendar/tests/action_event_test.php
+++ b/calendar/tests/action_event_test.php
@@ -200,7 +200,7 @@ class core_calendar_action_event_test_event_collection implements event_collecti
         return 2;
     }
 
-    public function getIterator() {
+    public function getIterator(): \Traversable {
         foreach ($this->events as $event) {
             yield $event;
         }
diff --git a/calendar/tests/event_mapper_test.php b/calendar/tests/event_mapper_test.php
index 0428de004a4..2fe720b51df 100644
--- a/calendar/tests/event_mapper_test.php
+++ b/calendar/tests/event_mapper_test.php
@@ -464,7 +464,7 @@ class core_calendar_event_mapper_test_event_collection implements event_collecti
         return 2;
     }
 
-    public function getIterator() {
+    public function getIterator(): \Traversable {
         foreach ($this->events as $event) {
             yield $event;
         }
diff --git a/calendar/tests/event_test.php b/calendar/tests/event_test.php
index 26abea7e6e7..b6cedc6e70f 100644
--- a/calendar/tests/event_test.php
+++ b/calendar/tests/event_test.php
@@ -136,7 +136,7 @@ class core_calendar_event_test_event_collection implements event_collection_inte
         return 2;
     }
 
-    public function getIterator() {
+    public function getIterator(): \Traversable {
         foreach ($this->events as $event) {
             yield $event;
         }
diff --git a/course/classes/category.php b/course/classes/category.php
index 9a62a0d2f2e..5fa9ebc5686 100644
--- a/course/classes/category.php
+++ b/course/classes/category.php
@@ -196,7 +196,7 @@ class core_course_category implements renderable, cacheable_object, IteratorAggr
      *
      * @return ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         $ret = array();
         foreach (self::$coursecatfields as $property => $unused) {
             if ($this->$property !== false) {
diff --git a/course/classes/list_element.php b/course/classes/list_element.php
index 5acafff2053..7f8f8137642 100644
--- a/course/classes/list_element.php
+++ b/course/classes/list_element.php
@@ -336,7 +336,7 @@ class core_course_list_element implements IteratorAggregate {
      *
      * @return ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         $ret = array('id' => $this->record->id);
         foreach ($this->record as $property => $value) {
             $ret[$property] = $value;
diff --git a/lib/accesslib.php b/lib/accesslib.php
index bf7e1f7d6ab..2ec5e3ca091 100644
--- a/lib/accesslib.php
+++ b/lib/accesslib.php
@@ -5217,7 +5217,7 @@ abstract class context extends stdClass implements IteratorAggregate {
      * Now we can convert context object to array using convert_to_array(),
      * and feed it properly to json_encode().
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         $ret = array(
             'id'           => $this->id,
             'contextlevel' => $this->contextlevel,
diff --git a/lib/classes/chart_axis.php b/lib/classes/chart_axis.php
index 1703472bad6..437accdf47a 100644
--- a/lib/classes/chart_axis.php
+++ b/lib/classes/chart_axis.php
@@ -129,6 +129,7 @@ class chart_axis implements JsonSerializable {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         return [
             'label' => $this->label,
diff --git a/lib/classes/chart_bar.php b/lib/classes/chart_bar.php
index aef8adf1c81..10431c758f5 100644
--- a/lib/classes/chart_bar.php
+++ b/lib/classes/chart_bar.php
@@ -43,6 +43,7 @@ class chart_bar extends chart_base {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         $data = parent::jsonSerialize();
         $data['horizontal'] = $this->get_horizontal();
diff --git a/lib/classes/chart_base.php b/lib/classes/chart_base.php
index 687a2748a75..3ff4d964d75 100644
--- a/lib/classes/chart_base.php
+++ b/lib/classes/chart_base.php
@@ -77,6 +77,7 @@ class chart_base implements JsonSerializable, renderable {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         global $CFG;
         return [
diff --git a/lib/classes/chart_line.php b/lib/classes/chart_line.php
index 5a55e0e98e6..ba27d10c700 100644
--- a/lib/classes/chart_line.php
+++ b/lib/classes/chart_line.php
@@ -42,6 +42,7 @@ class chart_line extends chart_base {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         $data = parent::jsonSerialize();
         $data['smooth'] = $this->get_smooth();
diff --git a/lib/classes/chart_pie.php b/lib/classes/chart_pie.php
index 1ad6e560f94..1bc2e657e2c 100644
--- a/lib/classes/chart_pie.php
+++ b/lib/classes/chart_pie.php
@@ -42,6 +42,7 @@ class chart_pie extends chart_base {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         $data = parent::jsonSerialize();
         $data['doughnut'] = $this->get_doughnut();
diff --git a/lib/classes/chart_series.php b/lib/classes/chart_series.php
index a493748ec6e..85d063f29d8 100644
--- a/lib/classes/chart_series.php
+++ b/lib/classes/chart_series.php
@@ -183,6 +183,7 @@ class chart_series implements JsonSerializable {
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         $data = [
             'label' => $this->label,
diff --git a/lib/classes/dml/recordset_walk.php b/lib/classes/dml/recordset_walk.php
index 3d560cf8fd2..db7867d3ac4 100644
--- a/lib/classes/dml/recordset_walk.php
+++ b/lib/classes/dml/recordset_walk.php
@@ -88,6 +88,7 @@ class recordset_walk implements \Iterator {
      *
      * @return mixed|bool The returned value type will depend on the callback.
      */
+    #[\ReturnTypeWillChange]
     public function current() {
 
         if (!$this->recordset->valid()) {
@@ -111,8 +112,8 @@ class recordset_walk implements \Iterator {
      *
      * @return void
      */
-    public function next() {
-        return $this->recordset->next();
+    public function next(): void {
+        $this->recordset->next();
     }
 
     /**
@@ -120,6 +121,7 @@ class recordset_walk implements \Iterator {
      *
      * @return int
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->recordset->key();
     }
@@ -133,7 +135,7 @@ class recordset_walk implements \Iterator {
      *
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         if (!$valid = $this->recordset->valid()) {
             $this->close();
         }
@@ -145,7 +147,7 @@ class recordset_walk implements \Iterator {
      *
      * @return void
      */
-    public function rewind() {
+    public function rewind(): void {
         // No rewind as it is not implemented in moodle_recordset.
         return;
     }
diff --git a/lib/classes/event/base.php b/lib/classes/event/base.php
index a0cf9f6b7bf..bf38916972a 100644
--- a/lib/classes/event/base.php
+++ b/lib/classes/event/base.php
@@ -984,7 +984,7 @@ abstract class base implements \IteratorAggregate {
      *
      * @return \ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): \Traversable {
         return new \ArrayIterator($this->data);
     }
 
diff --git a/lib/dml/moodle_recordset.php b/lib/dml/moodle_recordset.php
index 975b253d79e..cd63defb825 100644
--- a/lib/dml/moodle_recordset.php
+++ b/lib/dml/moodle_recordset.php
@@ -58,7 +58,7 @@ abstract class moodle_recordset implements Iterator {
      * Rewinds are not supported!
      * @return void
      */
-    public function rewind() {
+    public function rewind(): void {
         // no seeking, sorry - let's ignore it ;-)
         return;
     }
diff --git a/lib/dml/mysqli_native_moodle_recordset.php b/lib/dml/mysqli_native_moodle_recordset.php
index 522eac5cacd..29ec49c2390 100644
--- a/lib/dml/mysqli_native_moodle_recordset.php
+++ b/lib/dml/mysqli_native_moodle_recordset.php
@@ -62,10 +62,12 @@ class mysqli_native_moodle_recordset extends moodle_recordset {
         return $row;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object)$this->current;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         // return first column value as key
         if (!$this->current) {
@@ -75,11 +77,11 @@ class mysqli_native_moodle_recordset extends moodle_recordset {
         return $key;
     }
 
-    public function next() {
+    public function next(): void {
         $this->current = $this->fetch_next();
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !empty($this->current);
     }
 
diff --git a/lib/dml/oci_native_moodle_recordset.php b/lib/dml/oci_native_moodle_recordset.php
index 55ba1af64d8..e4b5fa7eff0 100644
--- a/lib/dml/oci_native_moodle_recordset.php
+++ b/lib/dml/oci_native_moodle_recordset.php
@@ -56,10 +56,12 @@ class oci_native_moodle_recordset extends moodle_recordset {
         return $row;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object)$this->current;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         // return first column value as key
         if (!$this->current) {
@@ -69,11 +71,11 @@ class oci_native_moodle_recordset extends moodle_recordset {
         return $key;
     }
 
-    public function next() {
+    public function next(): void {
         $this->current = $this->fetch_next();
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !empty($this->current);
     }
 
diff --git a/lib/dml/pdo_moodle_recordset.php b/lib/dml/pdo_moodle_recordset.php
index 673d91f30b1..26e6a9ffa54 100644
--- a/lib/dml/pdo_moodle_recordset.php
+++ b/lib/dml/pdo_moodle_recordset.php
@@ -55,10 +55,12 @@ class pdo_moodle_recordset extends moodle_recordset {
         return $row;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object)$this->current;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         // return first column value as key
         if (!$this->current) {
@@ -68,11 +70,11 @@ class pdo_moodle_recordset extends moodle_recordset {
         return $key;
     }
 
-    public function next() {
+    public function next(): void {
         $this->current = $this->fetch_next();
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !empty($this->current);
     }
 
diff --git a/lib/dml/pgsql_native_moodle_recordset.php b/lib/dml/pgsql_native_moodle_recordset.php
index 6679934424f..6558651dab5 100644
--- a/lib/dml/pgsql_native_moodle_recordset.php
+++ b/lib/dml/pgsql_native_moodle_recordset.php
@@ -133,10 +133,12 @@ class pgsql_native_moodle_recordset extends moodle_recordset {
         return $row;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object)$this->current;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         // return first column value as key
         if (!$this->current) {
@@ -146,11 +148,11 @@ class pgsql_native_moodle_recordset extends moodle_recordset {
         return $key;
     }
 
-    public function next() {
+    public function next(): void {
         $this->current = $this->fetch_next();
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !empty($this->current);
     }
 
diff --git a/lib/dml/sqlsrv_native_moodle_recordset.php b/lib/dml/sqlsrv_native_moodle_recordset.php
index 0bbd85d43d1..3f8621679d6 100644
--- a/lib/dml/sqlsrv_native_moodle_recordset.php
+++ b/lib/dml/sqlsrv_native_moodle_recordset.php
@@ -105,10 +105,12 @@ class sqlsrv_native_moodle_recordset extends moodle_recordset {
         return $row;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object)$this->current;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         // return first column value as key
         if (!$this->current) {
@@ -118,7 +120,7 @@ class sqlsrv_native_moodle_recordset extends moodle_recordset {
         return $key;
     }
 
-    public function next() {
+    public function next(): void {
         if ($this->buffer === null) {
             $this->current = $this->fetch_next();
         } else {
@@ -126,7 +128,7 @@ class sqlsrv_native_moodle_recordset extends moodle_recordset {
         }
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !empty($this->current);
     }
 
diff --git a/lib/dml/tests/fixtures/read_slave_moodle_database_special.php b/lib/dml/tests/fixtures/read_slave_moodle_database_special.php
index be09eba0b43..3a8b0d775e2 100644
--- a/lib/dml/tests/fixtures/read_slave_moodle_database_special.php
+++ b/lib/dml/tests/fixtures/read_slave_moodle_database_special.php
@@ -106,28 +106,31 @@ class read_slave_moodle_recordset_special extends \moodle_recordset {
     }
     /**
      * Iterator interface
-     * @return stdClass
+     * @return \stdClass
      */
+    #[\ReturnTypeWillChange]
     public function current() {
-        return new stdClass();
+        return new \stdClass();
     }
     /**
      * Iterator interface
      * @return void
      */
+    #[\ReturnTypeWillChange]
     public function next() {
     }
     /**
      * Iterator interface
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
     }
     /**
      * Iterator interface
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         return false;
     }
 }
diff --git a/lib/dml/tests/fixtures/read_slave_moodle_recordset_special.php b/lib/dml/tests/fixtures/read_slave_moodle_recordset_special.php
index 452385f557d..8e4b6fdc077 100644
--- a/lib/dml/tests/fixtures/read_slave_moodle_recordset_special.php
+++ b/lib/dml/tests/fixtures/read_slave_moodle_recordset_special.php
@@ -44,6 +44,7 @@ class read_slave_moodle_recordset_special extends moodle_recordset {
      * Iterator interface
      * @return stdClass
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         return new stdClass();
     }
@@ -51,19 +52,20 @@ class read_slave_moodle_recordset_special extends moodle_recordset {
      * Iterator interface
      * @return void
      */
-    public function next() {
+    public function next(): void {
     }
     /**
      * Iterator interface
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
     }
     /**
      * Iterator interface
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         return false;
     }
 }
diff --git a/lib/filestorage/zip_archive.php b/lib/filestorage/zip_archive.php
index 4c5953b342c..1aa4673f313 100644
--- a/lib/filestorage/zip_archive.php
+++ b/lib/filestorage/zip_archive.php
@@ -364,7 +364,7 @@ class zip_archive extends file_archive {
      *
      * @return int number of files
      */
-    public function count() {
+    public function count(): int {
         if (!isset($this->za)) {
             return false;
         }
@@ -509,6 +509,7 @@ class zip_archive extends file_archive {
      *
      * @return stdClass
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         if (!isset($this->za)) {
             return false;
@@ -522,6 +523,7 @@ class zip_archive extends file_archive {
      *
      * @return int current file index
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->pos;
     }
@@ -529,14 +531,14 @@ class zip_archive extends file_archive {
     /**
      * Moves forward to next file.
      */
-    public function next() {
+    public function next(): void {
         $this->pos++;
     }
 
     /**
      * Rewinds back to the first file.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->pos = 0;
     }
 
@@ -545,7 +547,7 @@ class zip_archive extends file_archive {
      *
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         if (!isset($this->za)) {
             return false;
         }
diff --git a/lib/modinfolib.php b/lib/modinfolib.php
index f0d5ce73a4b..cf9d0c3049b 100644
--- a/lib/modinfolib.php
+++ b/lib/modinfolib.php
@@ -1568,7 +1568,7 @@ class cm_info implements IteratorAggregate {
      *
      * @return ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         // Make sure dynamic properties are retrieved prior to view properties.
         $this->obtain_dynamic_data();
         $ret = array();
@@ -3207,7 +3207,7 @@ class section_info implements IteratorAggregate {
      *
      * @return ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         $ret = array();
         foreach (get_object_vars($this) as $key => $value) {
             if (substr($key, 0, 1) == '_') {
diff --git a/lib/navigationlib.php b/lib/navigationlib.php
index 806328082fc..2209a3efaf6 100644
--- a/lib/navigationlib.php
+++ b/lib/navigationlib.php
@@ -1243,7 +1243,7 @@ class navigation_node_collection implements IteratorAggregate, Countable {
      *
      * @return int
      */
-    public function count() {
+    public function count(): int {
         return $this->count;
     }
     /**
@@ -1254,7 +1254,7 @@ class navigation_node_collection implements IteratorAggregate, Countable {
      *
      * @return ArrayIterator
      */
-    public function getIterator() {
+    public function getIterator(): Traversable {
         return new ArrayIterator($this->collection);
     }
 }
diff --git a/lib/table/classes/local/filter/filter.php b/lib/table/classes/local/filter/filter.php
index 2c9b94bcc5a..29a0ff0699f 100644
--- a/lib/table/classes/local/filter/filter.php
+++ b/lib/table/classes/local/filter/filter.php
@@ -104,6 +104,7 @@ class filter implements Countable, Iterator, JsonSerializable {
     /**
      * Return the current filter value.
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         if ($this->iteratorposition === null) {
             $this->rewind();
@@ -121,6 +122,7 @@ class filter implements Countable, Iterator, JsonSerializable {
      *
      * @return int
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         if ($this->iteratorposition === null) {
             $this->rewind();
@@ -258,6 +260,7 @@ class filter implements Countable, Iterator, JsonSerializable {
      *
      * @return mixed|object
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         return (object) [
             'name' => $this->get_name(),
diff --git a/lib/table/classes/local/filter/filterset.php b/lib/table/classes/local/filter/filterset.php
index be86f881a18..798d5454908 100644
--- a/lib/table/classes/local/filter/filterset.php
+++ b/lib/table/classes/local/filter/filterset.php
@@ -295,6 +295,7 @@ abstract class filterset implements JsonSerializable {
      *
      * @return mixed|object
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize() {
         return (object) [
             'jointype' => $this->get_join_type(),
diff --git a/lib/tests/completionlib_test.php b/lib/tests/completionlib_test.php
index ecd75696b2f..6d30cafaf71 100644
--- a/lib/tests/completionlib_test.php
+++ b/lib/tests/completionlib_test.php
@@ -1933,23 +1933,25 @@ class core_completionlib_fake_recordset implements Iterator {
         $this->index = 0;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return $this->values[$this->index];
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->values[$this->index];
     }
 
-    public function next() {
+    public function next(): void {
         $this->index++;
     }
 
-    public function rewind() {
+    public function rewind(): void {
         $this->index = 0;
     }
 
-    public function valid() {
+    public function valid(): bool {
         return count($this->values) > $this->index;
     }
 
diff --git a/lib/xmlize.php b/lib/xmlize.php
index 78cf82b2a87..c60fa3f9dfc 100644
--- a/lib/xmlize.php
+++ b/lib/xmlize.php
@@ -42,8 +42,6 @@ defined('MOODLE_INTERNAL') || die();
 class xml_format_exception extends moodle_exception {
     /** @var string */
     public $errorstring;
-    /** @var int */
-    public $line;
     /** @var char */
     public $char;
     /**
diff --git a/mod/data/classes/search/sortedcontentqueue.php b/mod/data/classes/search/sortedcontentqueue.php
index 12a555b47c5..2cbdc050662 100644
--- a/mod/data/classes/search/sortedcontentqueue.php
+++ b/mod/data/classes/search/sortedcontentqueue.php
@@ -56,9 +56,9 @@ class sortedcontentqueue extends \SPLPriorityQueue {
      *
      * @param int $key1
      * @param int $key2
-     * @return bool
+     * @return int
      */
-    public function compare($key1 , $key2) {
+    public function compare($key1 , $key2): int {
         $record1 = $this->contents[$key1];
         $record2 = $this->contents[$key2];
 
diff --git a/privacy/classes/local/request/contextlist_base.php b/privacy/classes/local/request/contextlist_base.php
index 41dc87e1e0d..1c48a03c99b 100644
--- a/privacy/classes/local/request/contextlist_base.php
+++ b/privacy/classes/local/request/contextlist_base.php
@@ -121,6 +121,7 @@ abstract class contextlist_base implements
      *
      * @return  \context
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         // It is possible that this context has been deleted and we now have subsequent calls being made with this
         // contextlist. Exceptions here will stop the further processing of this component and that is why we are
@@ -137,7 +138,7 @@ abstract class contextlist_base implements
                 $context = $this->current();
             } else {
                 // There are no more context ids left.
-                return;
+                return null;
             }
         }
         return $context;
@@ -148,6 +149,7 @@ abstract class contextlist_base implements
      *
      * @return  mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->iteratorposition;
     }
@@ -155,7 +157,7 @@ abstract class contextlist_base implements
     /**
      * Move to the next context in the list.
      */
-    public function next() {
+    public function next(): void {
         ++$this->iteratorposition;
     }
 
@@ -164,7 +166,7 @@ abstract class contextlist_base implements
      *
      * @return  bool
      */
-    public function valid() {
+    public function valid(): bool {
         return isset($this->contextids[$this->iteratorposition]);
     }
 
@@ -174,14 +176,14 @@ abstract class contextlist_base implements
      * The list of contexts is uniqued during the rewind.
      * The rewind is called at the start of most iterations.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->iteratorposition = 0;
     }
 
     /**
      * Return the number of contexts.
      */
-    public function count() {
+    public function count(): int {
         return count($this->contextids);
     }
 }
diff --git a/privacy/classes/local/request/contextlist_collection.php b/privacy/classes/local/request/contextlist_collection.php
index 722b541404b..cefd36c2d76 100644
--- a/privacy/classes/local/request/contextlist_collection.php
+++ b/privacy/classes/local/request/contextlist_collection.php
@@ -117,6 +117,7 @@ class contextlist_collection implements \Iterator, \Countable {
      *
      * @return  \context
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         $key = $this->get_key_from_position();
         return $this->contextlists[$key];
@@ -127,6 +128,7 @@ class contextlist_collection implements \Iterator, \Countable {
      *
      * @return  mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->get_key_from_position();
     }
@@ -134,7 +136,7 @@ class contextlist_collection implements \Iterator, \Countable {
     /**
      * Move to the next context in the list.
      */
-    public function next() {
+    public function next(): void {
         ++$this->iteratorposition;
     }
 
@@ -143,7 +145,7 @@ class contextlist_collection implements \Iterator, \Countable {
      *
      * @return  bool
      */
-    public function valid() {
+    public function valid(): bool {
         return ($this->iteratorposition < count($this->contextlists));
     }
 
@@ -153,7 +155,7 @@ class contextlist_collection implements \Iterator, \Countable {
      * The list of contexts is uniqued during the rewind.
      * The rewind is called at the start of most iterations.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->iteratorposition = 0;
     }
 
@@ -174,7 +176,7 @@ class contextlist_collection implements \Iterator, \Countable {
     /**
      * Return the number of contexts.
      */
-    public function count() {
+    public function count(): int {
         return count($this->contextlists);
     }
 }
diff --git a/privacy/classes/local/request/userlist_base.php b/privacy/classes/local/request/userlist_base.php
index 3a49a222276..0e2566e6b61 100644
--- a/privacy/classes/local/request/userlist_base.php
+++ b/privacy/classes/local/request/userlist_base.php
@@ -146,6 +146,7 @@ abstract class userlist_base implements
      *
      * @return  \user
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         $user = \core_user::get_user($this->userids[$this->iteratorposition]);
 
@@ -160,7 +161,7 @@ abstract class userlist_base implements
                 $user = $this->current();
             } else {
                 // There are no more context ids left.
-                return;
+                return null;
             }
         }
 
@@ -172,6 +173,7 @@ abstract class userlist_base implements
      *
      * @return  mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->iteratorposition;
     }
@@ -179,7 +181,7 @@ abstract class userlist_base implements
     /**
      * Move to the next user in the list.
      */
-    public function next() {
+    public function next(): void {
         ++$this->iteratorposition;
     }
 
@@ -188,7 +190,7 @@ abstract class userlist_base implements
      *
      * @return  bool
      */
-    public function valid() {
+    public function valid(): bool {
         return isset($this->userids[$this->iteratorposition]) && $this->current();
     }
 
@@ -198,14 +200,14 @@ abstract class userlist_base implements
      * The list of users is uniqued during the rewind.
      * The rewind is called at the start of most iterations.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->iteratorposition = 0;
     }
 
     /**
      * Return the number of users.
      */
-    public function count() {
+    public function count(): int {
         return count($this->userids);
     }
 
diff --git a/privacy/classes/local/request/userlist_collection.php b/privacy/classes/local/request/userlist_collection.php
index 72eff15ea57..1438e94ea8a 100644
--- a/privacy/classes/local/request/userlist_collection.php
+++ b/privacy/classes/local/request/userlist_collection.php
@@ -114,6 +114,7 @@ class userlist_collection implements \Iterator, \Countable {
      *
      * @return  \user
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         $key = $this->get_key_from_position();
         return $this->userlists[$key];
@@ -124,6 +125,7 @@ class userlist_collection implements \Iterator, \Countable {
      *
      * @return  mixed
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->get_key_from_position();
     }
@@ -131,7 +133,7 @@ class userlist_collection implements \Iterator, \Countable {
     /**
      * Move to the next user in the list.
      */
-    public function next() {
+    public function next(): void {
         ++$this->iteratorposition;
     }
 
@@ -140,7 +142,7 @@ class userlist_collection implements \Iterator, \Countable {
      *
      * @return  bool
      */
-    public function valid() {
+    public function valid(): bool {
         return ($this->iteratorposition < count($this->userlists));
     }
 
@@ -150,7 +152,7 @@ class userlist_collection implements \Iterator, \Countable {
      * The list of users is uniqued during the rewind.
      * The rewind is called at the start of most iterations.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->iteratorposition = 0;
     }
 
@@ -171,7 +173,7 @@ class userlist_collection implements \Iterator, \Countable {
     /**
      * Return the number of users.
      */
-    public function count() {
+    public function count(): int {
         return count($this->userlists);
     }
 }
diff --git a/question/engine/questionattempt.php b/question/engine/questionattempt.php
index 097465c7a42..31ba3235c6b 100644
--- a/question/engine/questionattempt.php
+++ b/question/engine/questionattempt.php
@@ -1814,36 +1814,39 @@ class question_attempt_step_iterator implements Iterator, ArrayAccess {
     }
 
     /** @return question_attempt_step */
+    #[\ReturnTypeWillChange]
     public function current() {
         return $this->offsetGet($this->i);
     }
     /** @return int */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->i;
     }
-    public function next() {
+    public function next(): void {
         ++$this->i;
     }
-    public function rewind() {
+    public function rewind(): void {
         $this->i = 0;
     }
     /** @return bool */
-    public function valid() {
+    public function valid(): bool {
         return $this->offsetExists($this->i);
     }
 
     /** @return bool */
-    public function offsetExists($i) {
+    public function offsetExists($i): bool {
         return $i >= 0 && $i < $this->qa->get_num_steps();
     }
     /** @return question_attempt_step */
+    #[\ReturnTypeWillChange]
     public function offsetGet($i) {
         return $this->qa->get_step($i);
     }
-    public function offsetSet($offset, $value) {
+    public function offsetSet($offset, $value): void {
         throw new coding_exception('You are only allowed read-only access to question_attempt::states through a question_attempt_step_iterator. Cannot set.');
     }
-    public function offsetUnset($offset) {
+    public function offsetUnset($offset): void {
         throw new coding_exception('You are only allowed read-only access to question_attempt::states through a question_attempt_step_iterator. Cannot unset.');
     }
 }
@@ -1857,11 +1860,11 @@ class question_attempt_step_iterator implements Iterator, ArrayAccess {
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class question_attempt_reverse_step_iterator extends question_attempt_step_iterator {
-    public function next() {
+    public function next(): void {
         --$this->i;
     }
 
-    public function rewind() {
+    public function rewind(): void {
         $this->i = $this->qa->get_num_steps() - 1;
     }
 }
@@ -1933,21 +1936,23 @@ class question_attempt_steps_with_submitted_response_iterator extends question_a
     }
 
     /** @return question_attempt_step */
+    #[\ReturnTypeWillChange]
     public function current() {
         return $this->offsetGet($this->submittedresponseno);
     }
     /** @return int */
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->submittedresponseno;
     }
-    public function next() {
+    public function next(): void {
         ++$this->submittedresponseno;
     }
-    public function rewind() {
+    public function rewind(): void {
         $this->submittedresponseno = 1;
     }
     /** @return bool */
-    public function valid() {
+    public function valid(): bool {
         return $this->submittedresponseno >= 1 && $this->submittedresponseno <= count($this->stepswithsubmittedresponses);
     }
 
@@ -1955,7 +1960,7 @@ class question_attempt_steps_with_submitted_response_iterator extends question_a
      * @param int $submittedresponseno
      * @return bool
      */
-    public function offsetExists($submittedresponseno) {
+    public function offsetExists($submittedresponseno): bool {
         return $submittedresponseno >= 1;
     }
 
@@ -1963,6 +1968,7 @@ class question_attempt_steps_with_submitted_response_iterator extends question_a
      * @param int $submittedresponseno
      * @return question_attempt_step
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($submittedresponseno) {
         if ($submittedresponseno > count($this->stepswithsubmittedresponses)) {
             return null;
@@ -1974,7 +1980,7 @@ class question_attempt_steps_with_submitted_response_iterator extends question_a
     /**
      * @return int the count of steps with tries.
      */
-    public function count() {
+    public function count(): int {
         return count($this->stepswithsubmittedresponses);
     }
 
@@ -1993,11 +1999,11 @@ class question_attempt_steps_with_submitted_response_iterator extends question_a
         }
     }
 
-    public function offsetSet($offset, $value) {
+    public function offsetSet($offset, $value): void {
         throw new coding_exception('You are only allowed read-only access to question_attempt::states '.
                                    'through a question_attempt_step_iterator. Cannot set.');
     }
-    public function offsetUnset($offset) {
+    public function offsetUnset($offset): void {
         throw new coding_exception('You are only allowed read-only access to question_attempt::states '.
                                    'through a question_attempt_step_iterator. Cannot unset.');
     }
diff --git a/question/engine/questionusage.php b/question/engine/questionusage.php
index 116eca571de..aaf4c86dc21 100644
--- a/question/engine/questionusage.php
+++ b/question/engine/questionusage.php
@@ -1056,6 +1056,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      *
      * @return question_attempt
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         return $this->offsetGet(current($this->slots));
     }
@@ -1065,6 +1066,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      *
      * @return int
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         return current($this->slots);
     }
@@ -1072,14 +1074,14 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
     /**
      * Standard part of the Iterator interface.
      */
-    public function next() {
+    public function next(): void {
         next($this->slots);
     }
 
     /**
      * Standard part of the Iterator interface.
      */
-    public function rewind() {
+    public function rewind(): void {
         reset($this->slots);
     }
 
@@ -1088,7 +1090,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      *
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         return current($this->slots) !== false;
     }
 
@@ -1098,7 +1100,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      * @param int $slot
      * @return bool
      */
-    public function offsetExists($slot) {
+    public function offsetExists($slot): bool {
         return in_array($slot, $this->slots);
     }
 
@@ -1108,6 +1110,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      * @param int $slot
      * @return question_attempt
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($slot) {
         return $this->quba->get_question_attempt($slot);
     }
@@ -1118,7 +1121,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      * @param int $slot
      * @param question_attempt $value
      */
-    public function offsetSet($slot, $value) {
+    public function offsetSet($slot, $value): void {
         throw new coding_exception('You are only allowed read-only access to ' .
                 'question_attempt::states through a question_attempt_step_iterator. Cannot set.');
     }
@@ -1128,7 +1131,7 @@ class question_attempt_iterator implements Iterator, ArrayAccess {
      *
      * @param int $slot
      */
-    public function offsetUnset($slot) {
+    public function offsetUnset($slot): void {
         throw new coding_exception('You are only allowed read-only access to ' .
                 'question_attempt::states through a question_attempt_step_iterator. Cannot unset.');
     }
diff --git a/question/engine/tests/helpers.php b/question/engine/tests/helpers.php
index a1abe623985..33d9894c01a 100644
--- a/question/engine/tests/helpers.php
+++ b/question/engine/tests/helpers.php
@@ -1365,10 +1365,12 @@ class question_test_recordset extends moodle_recordset {
         $this->close();
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return (object) current($this->records);
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         if (is_null(key($this->records))) {
             return false;
@@ -1377,11 +1379,11 @@ class question_test_recordset extends moodle_recordset {
         return reset($current);
     }
 
-    public function next() {
+    public function next(): void {
         next($this->records);
     }
 
-    public function valid() {
+    public function valid(): bool {
         return !is_null(key($this->records));
     }
 
diff --git a/search/classes/skip_future_documents_iterator.php b/search/classes/skip_future_documents_iterator.php
index 6297fd80503..1a7d6a8faf1 100644
--- a/search/classes/skip_future_documents_iterator.php
+++ b/search/classes/skip_future_documents_iterator.php
@@ -67,6 +67,7 @@ class skip_future_documents_iterator implements \Iterator {
         }
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         if (!$this->gotcurrent) {
             $this->currentdoc = $this->parent->current();
@@ -75,16 +76,17 @@ class skip_future_documents_iterator implements \Iterator {
         return $this->currentdoc;
     }
 
-    public function next() {
+    public function next(): void {
         $this->parent->next();
         $this->gotcurrent = false;
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         return $this->parent->key();
     }
 
-    public function valid() {
+    public function valid(): bool {
         // Check that the parent is valid.
         if (!$this->parent->valid()) {
             return false;
@@ -99,7 +101,7 @@ class skip_future_documents_iterator implements \Iterator {
         }
     }
 
-    public function rewind() {
+    public function rewind(): void {
         $this->parent->rewind();
         $this->gotcurrent = false;
     }
diff --git a/search/tests/skip_future_documents_iterator_test.php b/search/tests/skip_future_documents_iterator_test.php
index 50ec9ebdf1d..a12ce76ac5b 100644
--- a/search/tests/skip_future_documents_iterator_test.php
+++ b/search/tests/skip_future_documents_iterator_test.php
@@ -202,6 +202,7 @@ class test_counting_iterator implements \Iterator {
      *
      * @return mixed Can return any type.
      */
+    #[\ReturnTypeWillChange]
     public function current() {
         $this->count++;
         return false;
@@ -219,7 +220,7 @@ class test_counting_iterator implements \Iterator {
     /**
      * Goes on to the next element.
      */
-    public function next() {
+    public function next(): void {
         $this->pos++;
     }
 
@@ -228,6 +229,7 @@ class test_counting_iterator implements \Iterator {
      *
      * @throws \coding_exception Always
      */
+    #[\ReturnTypeWillChange]
     public function key() {
         throw new \coding_exception('Unsupported');
     }
@@ -237,14 +239,14 @@ class test_counting_iterator implements \Iterator {
      *
      * @return bool True if still valid
      */
-    public function valid() {
+    public function valid(): bool {
         return $this->pos < 3;
     }
 
     /**
      * Rewinds the iterator.
      */
-    public function rewind() {
+    public function rewind(): void {
         $this->pos = 0;
     }
 }