mcrypt encode and decode

I am using codeigniter 2x, and I need to decode string I have been given, and that string containes array. For that I got a key, iv, cipher and mode.
I figure out how to set key, cipher and mode:

Code:

$this->load->library('encrypt');
$this->encrypt->set_cipher(MCRYPT_RIJNDAEL_128);
$this->encrypt->set_mode(MCRYPT_MODE_CBC);
$key = 'mysomekey';

But I can't figure out how to use iv I have been give, and decode that string.
On net I did found only this:

Code:

$o = $this->encrypt->decode($o,$key);

Can anyone helps me?