PHP string encryption enhanced version

/*
* 字符串加解密类；
* 一次一密；且定时解密有效
* 可用于加密&动态key生成
 */
class Mcrypt{
  public $default_key = 'a!takA:dlmcldEv,e';

/**
 * 字符加密，一次一密,可定时解密有效
 * @param string $string 原文
 * @param string $key 密钥
 * @param int $expiry 密文有效期,单位s,0 为永久有效
 * @return string 加密后的内容
*/
public static function encode($string,$key = '', $expiry = 0){
    $ckeyLength = 4;
    $key = md5($key ? $key : $this->default_key); //解密密匙
    $keya = md5(substr($key, 0, 16));         //做数据完整性验证  
    $keyb = md5(substr($key, 16, 16));         //用于变化生成的密文 (初始化向量IV)
    $keyc = substr(md5(microtime()), - $ckeyLength);
    $cryptkey = $keya . md5($keya . $keyc);  
    $keyLength = strlen($cryptkey);
    $string = sprintf('%010d', $expiry ? $expiry + time() : 0).substr(md5($string . $keyb), 0, 16) . $string;
    $stringLength = strlen($string);

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

    $box = range(0, 255);    
    // 打乱密匙簿，增加随机性
    for($j = $i = 0; $i < 256; $i++) {
        $j = ($j + $box[$i] + $rndkey[$i]) % 256;
        $tmp = $box[$i];
        $box[$i] = $box[$j];
        $box[$j] = $tmp;
     }    
     // 加解密，从密匙簿得出密匙进行异或，再转成字符
     $result = &#39;&#39;;
     for($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]));
      }
     $result = $keyc . str_replace(&#39;=&#39;, &#39;&#39;, base64_encode($result));
     $result = str_replace(array(&#39;+&#39;, &#39;/&#39;, &#39;=&#39;),array(&#39;-&#39;, &#39;_&#39;, &#39;.&#39;), $result);
     return $result;
 }

 /**
 * 字符解密，一次一密,可定时解密有效 
 * @param string $string 密文
 * @param string $key 解密密钥
 * @return string 解密后的内容
 */
 public static function decode($string,$key = &#39;&#39;) {
       $string = str_replace(array(&#39;-&#39;, &#39;_&#39;, &#39;.&#39;),array(&#39;+&#39;, &#39;/&#39;, &#39;=&#39;), $string);
       $ckeyLength = 4;
       $key = md5($key ? $key : $this->default_key); //解密密匙
       $keya = md5(substr($key, 0, 16));         //做数据完整性验证  
       $keyb = md5(substr($key, 16, 16));         //用于变化生成的密文 (初始化向量IV)
       $keyc = substr($string, 0, $ckeyLength);
       $cryptkey = $keya . md5($keya . $keyc);  
       $keyLength = strlen($cryptkey);
       $string = base64_decode(substr($string, $ckeyLength));
       $stringLength = strlen($string);

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

        $box = range(0, 255);
        // 打乱密匙簿，增加随机性
        for($j = $i = 0; $i < 256; $i++) {
              $j = ($j + $box[$i] + $rndkey[$i]) % 256;
              $tmp = $box[$i];
              $box[$i] = $box[$j];
              $box[$j] = $tmp;
        }
       // 加解密，从密匙簿得出密匙进行异或，再转成字符
       $result = &#39;&#39;;
       for($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 ((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 &#39;&#39;;
      } 
   }
}