gistfile1.php

<?php

   public function encrypt( $plaintext )
   {
      $iv        = mcrypt_create_iv( $this->iv_size, MCRYPT_RAND );
      $encrypted = mcrypt_encrypt( MCRYPT_RIJNDAEL_128, $this->key, $plaintext, MCRYPT_MODE_CBC, $iv );
      
      return base64_encode( $iv . $encrypted );
   }

   /**
    * decrypts a string. assumes the intialization vector is prepended to
    * the ciphertext
    * 
    * @param  [type] $cipher [description]
    *
    * @return mixed
    * returns a decoded string, or false
    */
   public function decrypt( $cipher )
   {
      $cipher = base64_decode( $cipher );
      $iv     = substr( $cipher, 0, $this->iv_size );
      $cipher = substr( $cipher, $this->iv_size );

      return mcrypt_decrypt( MCRYPT_RIJNDAEL_128, $this->key, $cipher, MCRYPT_MODE_CBC, $iv );
   }