In web page
http://metexplore.toulouse.inra.fr/metexplore/index.php/2011-06-06-14-41- 10/choke-point-reactions there exist a form with form id="form" but no name and no number. So i have used form_id function of mechanize module
<form id="form" action="printChokePoints.php" method="POST" enctype="multipart/form-data">