php function get age from birthday

birthday format is YYYY-MM-DD

function age($birthDay) {
    list($y, $m, $d) = explode('-', $birthDay);
    if (($m = (date('m') - $m)) < 0) {
        $y++;
    } elseif ($m == 0 && date('d') - $d < 0) {
        $y++;
    }
  
    return date('Y') - $y;
}