I added this to postbit_display_complete and it worked just fine (and added $testing to my postbit):
PHP Code:
$array = array("Test 1","Test 2");
shuffle($array);
$testing = $array[0];
So, I'd say your condition is wrong. First off, $user is most likely not valid. If this is the person who is posting, then that would be $this->post['userid'] . And, you should probably be using $post['field5'], not $post[field5].