[eluser]CroNiX[/eluser]
The callback function will only receive 2 parameters. The first is the value of the form field that was submitted and the second is a STRING of whatever is in the brackets of a callback (if you use brackets).
Code:
function compare_travel_dates($submitted_date, $everything_in_the_brackets){}
So I'm not sure how you plan on using 3 different dates in there (from_travel_date, $until_travel_date, $earliest_date)...
Unless they are different form fields, in which case they wouldn't get passed to your val function. Within the function you would:
Code:
function compare_travel_dates($submitted_date)
{
$until_travel_date = $this->input->post('travel_until_date'); //or whatever its called
$earliest_date = $this->input->post('earliest_date');
//do your comparisons against $submitted_date
//...
//set the error message
$this->form_validation->set_message('compare_travel_dates', 'Incorrect dates');
//return pass or fail rule
return TRUE/FALSE;
}