diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 6a60ce9..79689c0 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -684,22 +684,36 @@ function simpletest_clean_database() {
 
 /**
  * Finds all leftover temporary directories and removes them.
+ *
+ * @param string $directory
+ *   (optional) The relative path to directory of a particular test site. Must
+ *   to be of the following pattern: sites/simpletest/12345678. If ommited, all
+ *   the test sites' directories found in sites/simpletest will be removed.
  */
 function simpletest_clean_temporary_directories($directory = NULL) {
   $count = 0;
   $root = \Drupal::root() . '/sites/simpletest/';
-  $dir = $directory ? explode('sites/simpletest/', $directory) : FALSE;
 
   if (is_dir($root)) {
-    $files = !$dir ? scandir($root) : (isset($dir[1]) && is_dir("{$root}{$dir[1]}") ? [$dir[1]] : []);
+    // Prevent removing wrong directory accidentally passed as an argument.
+    $directory = $directory ? explode('sites/simpletest/', $directory) : FALSE;
+    $files = [];
+
+    if (isset($directory[1]) && is_dir($root . $directory[1])) {
+      $files[] = $directory[1];
+    }
+    else {
+      $files = scandir($root);
+    }
+
     foreach ($files as $file) {
       if ($file[0] != '.') {
-        $path = "{$root}{$file}";
+        $path = $root . $file;
+        // When the webserver runs with the same system user as the test
+        // runner, we can make read-only files writable again. If not, chmod
+        // will fail while the file deletion still works if file permissions
+        // have been configured correctly. Thus, we ignore any chmod errors.
         $result = file_unmanaged_delete_recursive($path, function ($any_path) {
-          // When the webserver runs with the same system user as the test
-          // runner, we can make read-only files writable again. If not, chmod
-          // will fail while the file deletion still works if file permissions
-          // have been configured correctly. Thus, we ignore any chmod errors.
           @chmod($any_path, 0700);
         });
         if ($directory) {