If you have the PEAR Date package installed (or can install it), you could use the Date_Calc class and not have to worry about OS-dependent timestamp restrictions:
require_once 'Date/Calc.php';
function isValidBirthDate($year, $month, $day)
{
$valid = false;
$dc = new Date_Calc();
if ($dc->isValidDate($day, $month, $year))
{
list($y, $m, $d) = explode('-', date('Y-m-d'));
if ($dc->compareDates($day, $month, $year, $d, $m, $y) <= 0)
{
$valid = true;
}
}
return $valid;
}