valid_date 500 error

[eluser]Darker[/eluser]

Code:

function valid_date($date)
{
  $birth_day = $this->input->post('birth_day');
  $birth_month = $this->input->post('birth_month');
  $birth_year = $this->input->post('birth_year');
  if(checkdate($birth_month, $birth_day, $birth_year))
  {
    return true;
  } else {
    return false;
  }
}

I'm getting 500 error when doin this in MY_Form_validation.php

Help please, thanks