public static function deleteAllFilesInDirectory ($dir) {
        $dirIterator = new \RecursiveDirectoryIterator($dir, \FilesystemIterator::SKIP_DOTS);
        foreach (new \RecursiveIteratorIterator($dirIterator, \RecursiveIteratorIterator::CHILD_FIRST) as $file) {
            $file->isDir() ? rmdir($file) : unlink($file);
        }
    }