diff --git a/lib/phpunit/classes/advanced_testcase.php b/lib/phpunit/classes/advanced_testcase.php
index a7a33bb7b59..e4bace9dce6 100644
--- a/lib/phpunit/classes/advanced_testcase.php
+++ b/lib/phpunit/classes/advanced_testcase.php
@@ -241,7 +241,7 @@ abstract class advanced_testcase extends PHPUnit_Framework_TestCase {
 
     /**
      * Return debugging messages from the current test.
-     * @return array
+     * @return array with instances having 'message', 'level' and 'stacktrace' property.
      */
     public function getDebuggingMessages() {
         return phpunit_util::get_debugging_messages();
@@ -249,10 +249,9 @@ abstract class advanced_testcase extends PHPUnit_Framework_TestCase {
 
     /**
      * Clear all previous debugging messages in current test.
-     * @return array
      */
     public function resetDebugging() {
-        return phpunit_util::reset_debugging();
+        phpunit_util::reset_debugging();
     }
 
     /**
@@ -293,6 +292,10 @@ abstract class advanced_testcase extends PHPUnit_Framework_TestCase {
         phpunit_util::reset_debugging();
     }
 
+    /**
+     * Call when no debugging() messages expected.
+     * @param string $message
+     */
     public function assertDebuggingNotCalled($message = '') {
         $debugging = phpunit_util::get_debugging_messages();
         $count = count($debugging);
diff --git a/lib/phpunit/classes/database_driver_testcase.php b/lib/phpunit/classes/database_driver_testcase.php
index 09b7dd2544b..4f56eb8a5e9 100644
--- a/lib/phpunit/classes/database_driver_testcase.php
+++ b/lib/phpunit/classes/database_driver_testcase.php
@@ -136,7 +136,7 @@ abstract class database_driver_testcase extends PHPUnit_Framework_TestCase {
 
     /**
      * Return debugging messages from the current test.
-     * @return array
+     * @return array with instances having 'message', 'level' and 'stacktrace' property.
      */
     public function getDebuggingMessages() {
         return phpunit_util::get_debugging_messages();
@@ -144,10 +144,9 @@ abstract class database_driver_testcase extends PHPUnit_Framework_TestCase {
 
     /**
      * Clear all previous debugging messages in current test.
-     * @return array
      */
     public function resetDebugging() {
-        return phpunit_util::reset_debugging();
+        phpunit_util::reset_debugging();
     }
 
     /**
@@ -188,6 +187,10 @@ abstract class database_driver_testcase extends PHPUnit_Framework_TestCase {
         phpunit_util::reset_debugging();
     }
 
+    /**
+     * Call when no debugging() messages expected.
+     * @param string $message
+     */
     public function assertDebuggingNotCalled($message = '') {
         $debugging = phpunit_util::get_debugging_messages();
         $count = count($debugging);
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index 34db5a016a0..61d350a3b6d 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -11,6 +11,11 @@ YUI changes:
 * moodle-enrol-notification has been renamed to moodle-core-notification
 * YUI2 code must now use 2in3, see http://yuilibrary.com/yui/docs/yui/yui-yui2.html
 
+Unit testing changes:
+* output debugging() is not sent to standard output any more,
+  use $this->assertDebuggingCalled(), $this->assertDebuggingNotCalled(),
+  $this->getDebuggingMessages() or $this->assertResetDebugging() instead.
+
 === 2.3 ===
 
 Database layer changes: