mcrypt_enc_get_iv_size(3) php man page | unix.com

MCRYPT_ENC_GET_IV_SIZE(3)						 1						 MCRYPT_ENC_GET_IV_SIZE(3)

mcrypt_enc_get_iv_size - Returns the size of the IV of the opened algorithm

SYNOPSIS
       int mcrypt_enc_get_iv_size (resource  $td)

DESCRIPTION
	This  function	returns the size of the IV of the algorithm specified by the encryption descriptor in bytes. An IV is used in cbc, cfb and
       ofb modes, and in some algorithms in stream mode.

PARAMETERS
	      o $td
		- The encryption descriptor.

RETURN VALUES
	Returns the size of the IV, or 0 if the IV is ignored by the algorithm.

PHP Documentation Group 												 MCRYPT_ENC_GET_IV_SIZE(3)