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mcrypt_enc_get_iv_size

Returns the size of the IV of the opened algorithm (PHP 4 >= 4.0.2, PHP 5)
int mcrypt_enc_get_iv_size ( resource td )

This function returns the size of the iv of the algorithm specified by the encryption descriptor in bytes. If it returns '0' then the IV is ignored in the algorithm. An IV is used in cbc, cfb and ofb modes, and in some algorithms in stream mode.