Problem with mbstring.internal_encoding, mcrypt and php-fpm

Hello,

from two days I have strange problem with this param 
mbstring.internal_encoding on nginx and php-fpm.
I use php version 5.4.20 and when I setting mbstring.internal_encoding I 
have a problem with mcrypt.

NOTICE: PHP message: PHP Warning:  mcrypt_generic_init(): Iv size 
incorrect; supplied length: 12, needed: 8 in file.php

I setup php.ini file:

zend.multibyte = On
default_mimetype = "text/html"
default_charset = "UTF-8"
mbstring.internal_encoding = UTF-8



and here is function:

  85         private function decrypt($blob)
  86         {
  87                 $deckey = $_SESSION['key'];
  88                 $realkey = sha1($deckey, true);
  89                 $rawblob = hex2bin($blob); /* binary blob */
  90                 $td = mcrypt_module_open($this->CIPHER, "", 
MCRYPT_MODE_CFB, "");
  91                 $iv = mb_substr($rawblob, 0, 
mcrypt_enc_get_iv_size($td)); /* IV */
  92                 if (mb_strlen($iv) < mcrypt_enc_get_iv_size($td)) 
return FALSE;
  93                 $ct = mb_substr($rawblob, 
mcrypt_enc_get_iv_size($td)); /* CipherText */
  94    --->         mcrypt_generic_init($td, $realkey, $iv);
  95                 $unblob = mdecrypt_generic($td, $ct);
  96                 mcrypt_generic_deinit($td);
  97                 $pt = mb_substr($unblob, 20);
  98                 $check = mb_substr($unblob, 0, 20);
  99                 if ($check != sha1($pt, true))
100                 {
101                         return FALSE;
102                 } else {
103                         return $pt;
104                 }
105         }


The same code with the same config options is worked fine with apache 
httpd.

Any one can give me a little help ?
Thanks

Regards,
Hristo S.