Validate Drop Down Selections

[eluser]Petsoukos[/eluser]
Hi,

I want to validate a date formed with drop down selections. I have three different drop down menus. (Year, Month, Day)
When it is submitted the controller receives three different POSTS. One for each drop down.

I can make the validation very easy with PHP checkdate() function:

Code:

$y = $this->input->post('y');
$m = $this->input->post('m');
$d = $this->input->post('d');
if(checkdate($m, $d, $y)) {
     return true;
} else {
     return false;
}

but if it fails how can I display the custom error message?