diff --git a/lib/mustache/readme_moodle.txt b/lib/mustache/readme_moodle.txt
index fc614724699..75b29f4ae75 100644
--- a/lib/mustache/readme_moodle.txt
+++ b/lib/mustache/readme_moodle.txt
@@ -20,3 +20,6 @@ from the list. If still not available upstream, they will need to be re-applied.
 
 - MDL-67114: PHP 7.4 compatibility. Array operations on scalar value.
   This corresponds to upstream https://github.com/bobthecow/mustache.php/pull/352
+- MDL-73586: PHP 8.0 compatibility. Removed 'mbstring.func_overload' init setting.
+  This corresponds to upstream commit https://github.com/bobthecow/mustache.php/commit/e7165a33b282ab4d20b3863825caadb46313d62b
+  that is availbale for the library versions 2.14.1 and up
diff --git a/lib/mustache/src/Mustache/Tokenizer.php b/lib/mustache/src/Mustache/Tokenizer.php
index 6dbe0cdfa03..71871226a34 100644
--- a/lib/mustache/src/Mustache/Tokenizer.php
+++ b/lib/mustache/src/Mustache/Tokenizer.php
@@ -97,11 +97,16 @@ class Mustache_Tokenizer
         // Setting mbstring.func_overload makes things *really* slow.
         // Let's do everyone a favor and scan this string as ASCII instead.
         //
+        // The INI directive was removed in PHP 8.0 so we don't need to check there (and can drop it
+        // when we remove support for older versions of PHP).
+        //
         // @codeCoverageIgnoreStart
         $encoding = null;
-        if (function_exists('mb_internal_encoding') && ini_get('mbstring.func_overload') & 2) {
-            $encoding = mb_internal_encoding();
-            mb_internal_encoding('ASCII');
+        if (version_compare(PHP_VERSION, '8.0.0', '<')) {
+            if (function_exists('mb_internal_encoding') && ini_get('mbstring.func_overload') & 2) {
+                $encoding = mb_internal_encoding();
+                mb_internal_encoding('ASCII');
+            }
         }
         // @codeCoverageIgnoreEnd
 
diff --git a/lib/phpunit/bootstrap.php b/lib/phpunit/bootstrap.php
index f3d04406fd9..e8be0752c2d 100644
--- a/lib/phpunit/bootstrap.php
+++ b/lib/phpunit/bootstrap.php
@@ -39,8 +39,6 @@ ini_set('log_errors', '1');
 if (ini_get('opcache.enable') and strtolower(ini_get('opcache.enable')) !== 'off') {
     if (!ini_get('opcache.save_comments') or strtolower(ini_get('opcache.save_comments')) === 'off') {
         ini_set('opcache.enable', 0);
-    } else {
-        ini_set('opcache.load_comments', 1);
     }
 }
 
diff --git a/mod/wiki/classes/external.php b/mod/wiki/classes/external.php
index 4a147559517..b51d41bcaf2 100644
--- a/mod/wiki/classes/external.php
+++ b/mod/wiki/classes/external.php
@@ -512,10 +512,14 @@ class mod_wiki_external extends external_api {
                     $retpage['contentformat'] = $contentformat;
                 } else {
                     // Return the size of the content.
-                    if (function_exists('mb_strlen') && ((int)ini_get('mbstring.func_overload') & 2)) {
-                        $retpage['contentsize'] = mb_strlen($cachedcontent, '8bit');
-                    } else {
-                        $retpage['contentsize'] = strlen($cachedcontent);
+                    $retpage['contentsize'] = strlen($cachedcontent);
+                    // TODO: Remove this block of code once PHP 8.0 is the min version supported.
+                    // For PHP < 8.0, if strlen() was overloaded, calculate
+                    // the bytes using mb_strlen(..., '8bit').
+                    if (PHP_VERSION_ID < 80000) {
+                        if (function_exists('mb_strlen') && ((int)ini_get('mbstring.func_overload') & 2)) {
+                            $retpage['contentsize'] = mb_strlen($cachedcontent, '8bit');
+                        }
                     }
                 }
 
diff --git a/mod/wiki/tests/externallib_test.php b/mod/wiki/tests/externallib_test.php
index 324f8d69d79..7e762a3fc0f 100644
--- a/mod/wiki/tests/externallib_test.php
+++ b/mod/wiki/tests/externallib_test.php
@@ -644,10 +644,14 @@ class externallib_test extends externallib_advanced_testcase {
 
         // Check that WS doesn't return page content if includecontent is false, it returns the size instead.
         foreach ($expectedpages as $i => $expectedpage) {
-            if (function_exists('mb_strlen') && ((int)ini_get('mbstring.func_overload') & 2)) {
-                $expectedpages[$i]['contentsize'] = mb_strlen($expectedpages[$i]['cachedcontent'], '8bit');
-            } else {
-                $expectedpages[$i]['contentsize'] = strlen($expectedpages[$i]['cachedcontent']);
+            $expectedpages[$i]['contentsize'] = strlen($expectedpages[$i]['cachedcontent']);
+            // TODO: Remove this block of code once PHP 8.0 is the min version supported.
+            // For PHP < 8.0, if strlen() was overloaded, calculate
+            // the bytes using mb_strlen(..., '8bit').
+            if (PHP_VERSION_ID < 80000) {
+                if (function_exists('mb_strlen') && ((int)ini_get('mbstring.func_overload') & 2)) {
+                    $expectedpages[$i]['contentsize'] = mb_strlen($expectedpages[$i]['cachedcontent'], '8bit');
+                }
             }
             unset($expectedpages[$i]['cachedcontent']);
             unset($expectedpages[$i]['contentformat']);