PHP Rename File If Exists

function rename_if_exists($dir, $filename) {

    $ext = strrchr($filename, '.');

    $prefix = substr($filename, 0, -strlen($ext));

    $i = 0;

    while(file_exists($dir . $filename)) { // If file exists, add a number to it.

        $filename = $prefix . ++$i . $ext;

    }

    return $filename;

}