Implementation skills related to PHP encryption and decryption

<?php
class Ender{
  private $enkey;//加密解密用的密钥
  private $rep_char=&#39;#&#39;;
  //替换加密后的base64字符串中的=,因为=在有些场合是禁止使用的，
  //这里可以用一个允许的字符作为替换。
  //构造参数是密钥
  public function __construct($key=&#39;&#39;){
    if(!$key){
      $this->enkey=$key;
    }
  }
  //设置密钥http://blog.ddian.cn
  public function set_key($key){
    $this->enkey=$key;
  }
  private function keyED($txt,$encrypt_key) 
  { 
  $encrypt_key = md5($encrypt_key); 
  $ctr=0; 
  $tmp = ""; 
  for ($i=0;$i<strlen($txt);$i++) 
  { 
  if ($ctr==strlen($encrypt_key)) $ctr=0; 
  $tmp.= substr($txt,$i,1) ^ substr($encrypt_key,$ctr,1); 
  $ctr++; 
  } 
  return $tmp;
  }
  //加密字符串
  public function encrypt($txt,$key=&#39;&#39;) 
  {
  if(!$key){
    $key=$this->enkey;
  }
  srand((double)microtime()*1000000); 
  $encrypt_key = md5(rand(0,32000)); 
  $ctr=0; 
  $tmp = ""; 
  for ($i=0;$i<strlen($txt);$i++) 
  { 
  if ($ctr==strlen($encrypt_key)) $ctr=0; 
  $tmp.= substr($encrypt_key,$ctr,1) . 
  (substr($txt,$i,1) ^ substr($encrypt_key,$ctr,1)); 
  $ctr++; 
  }
  $r=base64_encode($this->keyED($tmp,$key));
  $r=str_replace(&#39;=&#39;,$this->rep_char,$r);
  return $r; 
  }
  //解密字符串
  public function decrypt($txt,$key=&#39;&#39;) 
  {
  $txt=str_replace($this->rep_char,&#39;=&#39;,$txt);
  $txt=base64_decode($txt);
  if(!$key){
      $key=$this->enkey;
  }
  $txt = $this->keyED($txt,$key); 
  $tmp = ""; 
  for ($i=0;$i<strlen($txt);$i++) 
  { 
  $md5 = substr($txt,$i,1); 
  $i++; 
  $tmp.= (substr($txt,$i,1) ^ $md5); 
  } 
  return $tmp; 
  }
}