Dates et php

<?php  
$date_formulaire = $_POST['date_formulaire']; // variable contenant la date provenant du formulaire  
$duree = 60*60*24*7; // durée d'une semaine  
$resultat = $date_formulaire + $duree;  

echo date('d/m/Y', $resultat); // affiche le résultat  
?> 

	public function create_multiple_ok() {
		if ($this->_submit_validate() === FALSE) {
			$this->create();
			return;
		}
		if ( $this->input->post('time_beg') >= $this->input->post('time_end')){
			$this->load->view('error_prive');
		}
		else {
			$date_end = $this->input->post('date_end');

			$a=$this->input->post('time_beg');
			$b=$this->input->post('time_end');
			list($heure1, $minute1)=preg_split("/:/", $a);
			list($heure2, $minute2)=preg_split("/:/", $b);
			$a = $heure1 . ":" . $minute1 . ":00";
			$b = $heure2 . ":" . $minute2 . ":00";

			$nextweek = $this->input->post('date');

			while ($nextweek <= $date_end){

				list($y, $m, $j)=preg_split("/-/", $nextweek);
				$a= mktime($y, $m, $j, date("Y"), date("m"), date("d"));

				$u = new availability();
				$u->position = $this->input->post('position');
				$u->date = $a;
				$u->time_beg = $a;
				$u->time_end = $b;
				$u->teachers_id_teacher = $this->input->post('teachers_id_teacher');
				$u->students_id_student = $this->input->post('students_id_student');
				$u->save();

				$nextweek = mktime($y, $m, $j, date("Y"), date("m"), date("d")+7);
					
			}
			$this->load->view('requestExecuted_prive');
		}
	}