A simple encryption and decryption code for php

class SysCrypt{
 private $crypt_key='https://www.ofstack.com';// The key 
 public function __construct($crypt_key){
  $this->crypt_key=$crypt_key;
 }
 public function encrypt($txt){
  srand((double)microtime()*1000000);
  $encrypt_key=md5(rand(0,32000));
  $ctr=0;
  $tmp='';
  for($i=0;$i<strlen($txt);$i++){
   $ctr=$ctr==strlen($encrypt_key)?0:$ctr;
   $tmp.=$encrypt_key[$ctr].($txt[$i]^$encrypt_key[$ctr++]);
  }
  return base64_encode(self::__key($tmp,$this->crypt_key));
 }
 public function decrypt($txt){
  $txt=self::__key(base64_decode($txt),$this->crypt_key);
  $tmp='';
  for($i=0;$i<strlen($txt);$i++){
   $md5=$txt[$i];
   $tmp.=$txt[++$i]^$md5;
  }
  return $tmp;
 }
 private function __key($txt,$encrypt_key){
  $encrypt_key=md5($encrypt_key);
  $ctr=0;
  $tmp='';
  for($i=0;$i<strlen($txt);$i++){
   $ctr=$ctr==strlen($encrypt_key)?0:$ctr;
   $tmp.=$txt[$i]^$encrypt_key[$ctr++];
  }
  return $tmp;
 }
 public function __destruct(){
  $this->crypt_key=NULL;
 }
}

Usage of this class:

$sc=new SysCrypt('https://www.ofstack.com');
$text='yhm.1234@163.com';
$test1=$sc->encrypt($text);
echo ' The original: ',$text;
echo '<br />';
echo ' Cipher: ',$test1;
echo "<br/>";
echo ' Decryption: ',$sc->decrypt($test1);

The output results are similar:

Original text: yhm.1234@163.com
Cipher: BS8DbFU6AioCNFFlVGZQMgRGBDUOYlEzBXoAZgo0Bjk =
Decryption: yhm.1234@163.com