PHP文本路径转换为链接文字

<?php
/**
* 文本路径转换为有链接的文字
* @param string $str 转换内容
* @return string
*/
function urlToLink($str) {
    $arr = array("www." => "http://www.");
    $str = strtr($str, $arr);
    $arr = array("http://http://" => "http://");
    $str = strtr($str, $arr);
    $str2 = explode("http://", $str);
    for ($n = 1; isset($str2[$n]); $n ++) {
        $str3 = explode(".", $str2[$n]);
        if (isset($str3[1])) {
            $str4 = explode("www.", $str2[$n]);
            if ((isset($str4[1]) && isset($str3[2])) || !isset($str4[1])) {
                $length = strlen($str2[$n]);
                for ($i = 0; $i <= $length; $i ++) {
                    //从空格断开
                    if (($i - 1) == strlen(trim(mb_substr($str2[$n], 0, $i, 'gb2312')))) {
                        $ii = $i - 1;
                        $url1 = mb_substr($str2[$n], 0, $ii, 'gb2312');
                        $url2 = mb_substr($str2[$n], $ii, $length, 'gb2312');
                        $url3 = "<a href=\"http://" . $url1 . "\" target=\"_blank\">http://" . $url1 . "</a>" . $url2;
                        break;
                    } 

                    //从出现汉字处断开
                    if ($i != strlen(mb_substr($str2[$n], 0, $i, 'gb2312'))) {
                        $ii = $i - 1;
                        $url1 = mb_substr($str2[$n], 0, $ii, 'gb2312');
                        $url2 = mb_substr($str2[$n], $ii, $length, 'gb2312');
                        $url3 = "<a href=\"http://" . $url1 . "\" target=\"_blank\">http://" . $url1 . "</a>" . $url2;
                        break;
                    }
                    if ($i == $length)
                        $url3 = "<a href=\"http://" . $str2[$n] . "\" target=\"_blank\">http://" . $str2[$n] . "</a>";
                }
            } else
                $url3 = "http://" . $str2[$n];
        } else
            $url3 = "http://" . $str2[$n];
        $url .= $url3;
    }
    if (substr($str, 0, 7) == "http://")
        $url = "<a href=\"http://$str2[0]\" target=\"_blank\">" . $str2[0] . "</a>" . $url;
    else
        $url = $str2[0] . $url;
    return $url;
}
?>