先发一下原始的加密代码，通过传入$operation = 'ENCODE'|'DECODE' 来实现加密和解密，操作就是$data=authcode($data);

function authcode($string,$operation='ENCODE') {
        if ($operation=='ENCODE'){
                $OutTxt = "";
                for ($x=0;$x<strlen($string);$x++) {
                        $nr = ord($string[$x]);
                        if ($nr < 128) {
                                $nr += 128;
                        }
                        elseif ($nr > 127) {
                                $nr -= 128;
                        }
                        $nr = 255 - $nr;
                        $OutTxt .= sprintf("%02x", $nr);
                }
                return $OutTxt;
        }else{
                $OutTxt = "";
                for ($x=0;$x<(strlen($string)/2);$x++) {
                        $nr = hexdec($string[$x * 2] . $string[($x * 2) + 1]);
                        $nr = 255 - $nr;
                        if ($nr < 128) {
                                $nr += 128;
                        }
                        elseif ($nr > 127) {
                                $nr -= 128;
                        }
                        $OutTxt .= chr($nr);
                }
                return $OutTxt;
        }
}

把加密解密代码改一下，就可以单独解密了

<form method="post" name="form1" action="">
<input type="text" name="password" value="要解密的密码">
<br><input type="submit" name="Submit" value="提 交"><br><br>
<?php
$string = $_POST['password'];
$oldpass = $string;
function authcode($string) {
$OutTxt = "";
for ($x=0;$x<(strlen($string)/2);$x++) {
        $nr = hexdec($string[$x * 2] . $string[($x * 2) + 1]);
        $nr = 255 - $nr;
        if ($nr < 128) {
                $nr += 128;
        }
        elseif ($nr > 127) {
                $nr -= 128;
        }
        $OutTxt .= chr($nr);
}
return $OutTxt;
}
echo $oldpass;
echo " 解密后得到明文为：";
echo authcode($string);
?>