gistfile1.php

<?php 
class DateTimeDeluxe extends DateTime {
	public function isFirst($daystring) {
		return $this -> diff(new DateTime("first ".$daystring." of ".strtolower($this -> format("F")))) -> days === 0;
	}
}
$newDate = new DateTimeDeluxe();
// premier lundi du mois ?
var_dump($newDate -> isFirst('monday'));

?>