计算字符串的相似承度

<?php
    /**
     * @company xingquan
     * @auther        BianJiang
     * @date     Sat Mar 29 08:48:20 CST 2008
     * @copyright     Shen Yang Xing Quan
     * @todo 计算字符串的相似承度
     * @desc        计算字符串的相似承度
     * @version     0.1
    */
    $str_1 = "aaaa";
    $str_2 = "aaab";
    $percent = 0;
    similar_text($str_1, $str_2, $percent);
    echo $percent;
?>