Help with template creation

[dbembibre]

Hi, im new to vbulletin.
I create a custom template/php file to make a registration form.
I have a problem if any required data is missing i need exit from the php code and return to template, but this not work.
A example

<?php

if ($REQUES['do']="blablalbla")
{

$user=$_POST['usuario'];
$address=$_POST['address'];

//My problem here
if ($empty($user))
{
$Message_To_Template="Username is required";
exit;
}

}

eval('$navbar = "' . fetch_template('navbar') . '";');

eval('print_output("' . fetch_template('REG') . '");');

If i put a exit in my code the template never load, i need make a exit or a similar way and permit the fetch_template code

Excuse my broken English