php: validate url

gistfile1.php

<?php
 public static function validate_url($str)
    {
        if (empty($str))
            return true;

        if (strpos($str, "http") === false) {
            return false;
        } else {
            return (!preg_match('/^(http|https|ftp)); //\/\/([A-Z0-9][A-Z0-9_-]*(?:\.[A-Z0-9][A-Z0-9_-]*)+)); //?(\d+)?\/?/i', $str)) ? FALSE : TRUE;
        }
    }
?>