经典的自定义加密/解密函数

<?php
/**
 * 加密/解密函数
 *
 * @param string  $string    要加密的明文或要解密的密文
 * @param string  $operation DECODE=解密|ENCODE或其它=加密
 * @param string  $key       加密/解密的密钥
 * @param int     $expiry    密文有效期，0表示永久有效
 * @return string 加密后的密文/解密后的明文
 */
function authcode($string, $operation = 'DECODE', $key = '', $expiry = 0)
{
    $ckeyLength = 4;

    $key = md5($key ? $key : 'default_key');

    $keyA = md5(substr($key, 0, 16));
    $keyB = md5(substr($key, 16, 16));
    $keyC = $ckeyLength ? $operation == 'DECODE' ? substr($string, 0, $ckeyLength) : substr(md5(microtime()) , -$ckeyLength) : '';

    $cryptKey = $keyA . md5($keyA . $keyC);
    $keyLength = strlen($cryptKey);
    $string = $operation == 'DECODE' ? base64_decode(substr($string, $ckeyLength)) : sprintf('%010d', $expiry ? $expiry + time() : 0) . substr(md5($string . $keyB) , 0, 16) . $string;
    $stringLength = strlen($string);

    $result = '';
    $box = range(0, 255);
    $rndkey = array();

    for ($i = 0; $i <= 255; $i++)
    {
        $rndkey[$i] = ord($cryptKey[$i % $keyLength]);
    }

    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 < $stringLength; $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 ($operation == 'DECODE')
    {
        if ((substr($result, 0, 10) == 0 || substr($result, 0, 10) - time() > 0) && substr($result, 10, 16) == substr(md5(substr($result, 26) . $keyB) , 0, 16))
        {
            return substr($result, 26);
        }
        else
        {
            return '';
        }
    }
    else
    {
        return $keyC . str_replace('=', '', base64_encode($result));
    }
}

$string = 'PHP是世界上最好の语言';
$key    = '这是密钥';

echo "原始的字符串：{$string}<br>" . PHP_EOL; // 原始的字符串：PHP是世界上最好の语言

// 加密操作
$ciphertext = authcode($string, 'ENCODE', $key);
echo "加密后的密文：{$ciphertext}<br>" . PHP_EOL; // 加密后的密文：61f0FauPLccfurlvf3cJCQuSeLVAeN1jENbNLZ6elFynE6Sm2nnTpH2fgXJBC98MHdWX+9bN51st82E

// 解密操作
$plaintext = authcode($ciphertext, 'DECODE', '这是密钥', $key);
echo "解密后的明文：{$plaintext}<br>" . PHP_EOL; // 解密后的明文：PHP是世界上最好の语言