rule #1 when working with form data:
All Form processing must occur on the forms action page.
the action of your form is to post to the 2012_enviar.php page:
<form id="form1" name="form1" method="post" action="2012_enviar.php">
so the server validation behavior needs to be applied to that page.
this is similar to the way the eCart checkout and confirm pages work.
see this thread for details: