Custom validation (callback) for blank field value

[eluser]eangkasa[/eluser]
this is really weird ... i have copied the code from the link that you gave me before hvalente13, but it doesn't work ... the callback still not getting called without putting "required" in front.

$rules["whateverfield"] = "callback__some_function";

function _some_function($the_parameter)
{
$this->validation->set_message('_some_function', "Some error message.");
return FALSE;
}

Basically, the code above is a part of my code snippet, it is a very simple to start with cuz it will always return FALSE "IF" the callback function is being called on the first place.