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<?php
function rc4 ($pwd, $data)//$pwd密钥　$data需加密字符串
        {
                $key[] ="";
                $box[] ="";

                $pwd_length = strlen($pwd);
                $data_length = strlen($data);

                for ($i = 0; $i < 256; $i++)
                {
                        $key[$i] = ord($pwd[$i % $pwd_length]);
                        $box[$i] = $i;
                }

                for ($j = $i = 0; $i < 256; $i++)
                {
                        $j = ($j + $box[$i] + $key[$i]) % 256;
                        $tmp = $box[$i];
                        $box[$i] = $box[$j];
                        $box[$j] = $tmp;
                }

                for ($a = $j = $i = 0; $i < $data_length; $i++)
                {
                        $a = ($a + 1) % 256;
                        $j = ($j + $box[$a]) % 256;

                        $tmp = $box[$a];
                        $box[$a] = $box[$j];
                        $box[$j] = $tmp;

                        $k = $box[(($box[$a] + $box[$j]) % 256)];
                        $cipher .= chr(ord($data[$i]) ^ $k);

                }

                return $cipher;

        }

//用法:
$pwd="aoyou"; //这里为密钥,解密过程要用到
$data="123"; //需要加密的数据
$result=rc4($pwd,$data);//得到加密后的数据
echo $result.'<br>';
//用得到的密文再次加密《提供一样的密钥》　就可以反解出明文了
$befor=rc4($pwd,$result);
echo $befor.'<br>';
?>