为什么 mcrypt_encrypt() 将二进制字符放在我的字符串末尾?

Here is a PHP demo script that encrypts and decrypts data:

<?

$encryptionkey = 'h8y2p9d1';

$card_nbr = "1234";
echo "original card_nbr: $card_nbr <br>
";

$card_nbr_encrypted=encrypt_data($card_nbr);
echo "card_nbr_encrypted: $card_nbr_encrypted <br>
";

$card_nbr_decrypted=decrypt_data($card_nbr_encrypted);
echo "card_nbr_decrypted: $card_nbr_decrypted <br>
";

$len=strlen($card_nbr_decrypted);
echo "length: $len <br>
";



function encrypt_data($text){
  global $encryptionkey;
  $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
  $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
  $encrypted_text = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $encryptionkey, $text, MCRYPT_MODE_ECB, $iv);
  return $encrypted_text;
}

function decrypt_data($text){
  global $encryptionkey;
  $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
  $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
  $decrypted_text = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $encryptionkey, $text, MCRYPT_MODE_ECB, $iv);
  return $decrypted_text;
}

?>

输出为:

original card_nbr: 1234
card_nbr_encrypted: vY¨(Z$<§G3-žÃ-Éù3Ý2Ê×rz¨VÛ
card_nbr_decrypted: 1234  (and 28 binary characters)
length: 32 

输出成功解密，但最后添加了 28 个二进制字符.在 Firefox 中查看 HTML 源代码时最容易看到这一点.32 的字符串长度也证明了这一点.有什么想法吗?