diff --git a/lib/deprecatedlib.php b/lib/deprecatedlib.php
index b58d1ee69bb..6301a5d4ff3 100644
--- a/lib/deprecatedlib.php
+++ b/lib/deprecatedlib.php
@@ -105,17 +105,21 @@ function session_kill_user($userid) {
     \core\session\manager::kill_user_sessions($userid);
 }
 
-/**
- * Session garbage collection
- * - verify timeout for all users
- * - kill sessions of all deleted users
- * - kill sessions of users with disabled plugins or 'nologin' plugin
- *
- * @deprecated since 2.6
- */
-function session_gc() {
-    debugging('session_gc() is deprecated, use \core\session\manager::gc() instead', DEBUG_DEVELOPER);
-    \core\session\manager::gc();
+// PHP 5.6 includes session_gc(), we cannot define it any more.
+if (!function_exists('session_gc')) {
+    /**
+     * Session garbage collection
+     * - verify timeout for all users
+     * - kill sessions of all deleted users
+     * - kill sessions of users with disabled plugins or 'nologin' plugin
+     *
+     * @deprecated since 2.6
+     */
+    function session_gc()
+    {
+        debugging('session_gc() is deprecated, use \core\session\manager::gc() instead', DEBUG_DEVELOPER);
+        \core\session\manager::gc();
+    }
 }
 
 /**