FirstDayOfMonth

Returns a full textual representation of the day of the week for the first day of the month for a valid month/year combination.

function FirstDayOfMonth($month, $year) {
    if (empty($month)) {
      $month = date('m');
   }
   if (empty($year)) {
      $year = date('Y');
   }
   $result = strtotime("{$year}-{$month}-01");
   return date('l', $result);
}