diff --git a/autoload.php b/autoload.php
index c295b2ea75..e61c472fb1 100644
--- a/autoload.php
+++ b/autoload.php
@@ -210,7 +210,7 @@ public static function determineLoadedClass($classesBeforeLoad, $classesAfterLoa
             // That way, at the end, only the "main" class just included will remain.
             $newClasses = array_reduce(
                 $newClasses,
-                function ($remaining, $current) {
+                static function ($remaining, $current) {
                     return array_diff($remaining, class_parents($current));
                 },
                 $newClasses
diff --git a/src/Reports/Summary.php b/src/Reports/Summary.php
index 14692cfeda..c76e4bc2e5 100644
--- a/src/Reports/Summary.php
+++ b/src/Reports/Summary.php
@@ -97,7 +97,7 @@ public function generate(
 
         uksort(
             $reportFiles,
-            function ($keyA, $keyB) {
+            static function ($keyA, $keyB) {
                 $pathPartsA = explode(DIRECTORY_SEPARATOR, $keyA);
                 $pathPartsB = explode(DIRECTORY_SEPARATOR, $keyB);
 
diff --git a/src/Util/Cache.php b/src/Util/Cache.php
index 9d8d35d0b2..0146f4cdce 100644
--- a/src/Util/Cache.php
+++ b/src/Util/Cache.php
@@ -107,7 +107,7 @@ public static function load(Ruleset $ruleset, Config $config)
         );
         $filter = new RecursiveCallbackFilterIterator(
             $di,
-            function ($file, $key, $iterator) {
+            static function ($file, $key, $iterator) {
                 // Skip non-php files.
                 $filename = $file->getFilename();
                 if ($file->isFile() === true && substr($filename, -4) !== '.php') {