php MD5加密解密

参考链接：http://www.cnblogs.com/cfinder010/p/3552077.html
<?php /*
*$string :需要加密解密的字符串
*$operation:判断是加密还是解密:E:加密 D:解密
*$key :加密的钥匙(密匙);
*/
function encrypt($string,$operation,$key=''){
    $src = array("/","+","=");
    $dist = array("_a","_b","_c");
    if($operation=='D'){
        $string = str_replace($dist,$src,$string);
    }
    $key=md5($key);
    $key_length=strlen($key);
    $string=$operation=='D'?base64_decode($string):substr(md5($string.$key),0,8).$string;
    $string_length=strlen($string);
    $rndkey=$box=array();
    $result='';
    for($i=0;$i<=255;$i++){
        $rndkey[$i]=ord($key[$i%$key_length]);
        $box[$i]=$i;
    }
    for($j=$i=0;$i<256;$i++){
        $j=($j+$box[$i]+$rndkey[$i])%256;
        $tmp=$box[$i];
        $box[$i]=$box[$j];
        $box[$j]=$tmp;
    }
    for($a=$j=$i=0;$i<$string_length;$i++){
        $a=($a+1)%256;
        $j=($j+$box[$a])%256;
        $tmp=$box[$a];
        $box[$a]=$box[$j];
        $box[$j]=$tmp;
        $result.=chr(ord($string[$i])^($box[($box[$a]+$box[$j])%256]));
    }
    if($operation=='D'){
        if(substr($result,0,8)==substr(md5(substr($result,8).$key),0,8)){ // www.jbxue.com
            return substr($result,8);
        }else{
            return'';
        }
    }else{
        $rdate = str_replace('=','',base64_encode($result));
        $rdate = str_replace($src,$dist,$rdate);
        return $rdate;
    }
}

$sign = 'abcd';//一般签名数据最好加上当前时间戳

$signE = encrypt($sign,"E");

echo $signE;

$signD = encrypt($signE,"D");

echo "\r\n".$signD;

注：只适用于MD5密钥方式加密，不适用于第三方加密文件加密