Hello, I'm hoping someone can help me with this.

I have 3 pages that I need to put into my framework, they are products, shopping cart and billing. At the moment they work perfectly fine. Here is a live example -

http://www.cems.uwe.ac.uk/~r4-george...g/products.php

Now, I have a framework for a whole website that I need to put these pages into. (http://www.cems.uwe.ac.uk/~r4-george/wp4/index.php) The index uses a switch statement to go between pages.

Here is the index.php

PHP Code:
<?php # index.php

/* 
 *    This is the main page.
 *    This page includes the configuration file, 
 *    the templates, and any content-specific modules.
 */

// Require the configuration file before any PHP code:
require_once ('./modules/config.inc.php');

// Validate what page to show:
if (isset($_GET['p'])) {
    
$p $_GET['p'];
} elseif (isset(
$_POST['p'])) { // Forms
    
$p $_POST['p'];
} else {
    
$p NULL;
}

// Determine what page to display:
switch ($p) {

    case 
'about':
        
$page 'about.inc.php';
        
$page_title 'About This Site Again';
        break;
        
    case 
'products':
        
$page 'products.inc.php';
        
$page_title 'Products on this site';
        break;

    case 
'this':
        
$page 'this.inc.php';
        
$page_title 'This is Another Page.';
        break;
    
    case 
'that':
        
$page 'that.inc.php';
        
$page_title 'That is Also a Page.';
        break;
    
    case 
'contact':
        
$page 'contact.inc.php';
        
$page_title 'Contact Us';
        break;
    
    case 
'search':
        
$page 'search.inc.php';
        
$page_title 'Search Results';
        break;
    
    
// Default is to include the main page.
    
default:
        
$page 'main.inc.php';
        
$page_title 'Site Home Page';
        break;
        
// End of main switch.

// Make sure the file exists:
if (!file_exists('./modules/' $page)) {
    
$page 'main.inc.php';
    
$page_title 'Site Home Page';
}

// Include the header file:
include_once ('./includes/header.inc');


echo 
"<div id=\"content\">";

// Include the content-specific module:
// $page is determined from the above switch.
include ('./modules/' $page);

// Include the footer file to complete the template:
include_once ('./includes/footer.inc');

?>
It uses the .inc.php files located in the modules folder to switch between pages.

Here is my products.inc.php -

PHP Code:
<?php # index.php

/* 
 *    This is the main page.
 *    This page includes the configuration file, 
 *    the templates, and any content-specific modules.
 */

// Require the configuration file before any PHP code:
require_once ('./modules/config.inc.php');

// Validate what page to show:
if (isset($_GET['p'])) {
    
$p $_GET['p'];
} elseif (isset(
$_POST['p'])) { // Forms
    
$p $_POST['p'];
} else {
    
$p NULL;
}

// Determine what page to display:
switch ($p) {

    case 
'about':
        
$page 'about.inc.php';
        
$page_title 'About This Site Again';
        break;
        
    case 
'products':
        
$page 'products.inc.php';
        
$page_title 'Products on this site';
        break;

    case 
'this':
        
$page 'this.inc.php';
        
$page_title 'This is Another Page.';
        break;
    
    case 
'that':
        
$page 'that.inc.php';
        
$page_title 'That is Also a Page.';
        break;
    
    case 
'contact':
        
$page 'contact.inc.php';
        
$page_title 'Contact Us';
        break;
    
    case 
'search':
        
$page 'search.inc.php';
        
$page_title 'Search Results';
        break;
    
    
// Default is to include the main page.
    
default:
        
$page 'main.inc.php';
        
$page_title 'Site Home Page';
        break;
        
// End of main switch.

// Make sure the file exists:
if (!file_exists('./modules/' $page)) {
    
$page 'main.inc.php';
    
$page_title 'Site Home Page';
}

// Include the header file:
include_once ('./includes/header.inc');


echo 
"<div id=\"content\">";

// Include the content-specific module:
// $page is determined from the above switch.
include ('./modules/' $page);

// Include the footer file to complete the template:
include_once ('./includes/footer.inc');

?>
This is EXACTLY the same code as the working example. The products get listed correctly but the problem I have is the 'Add to Cart' button fails to work.

Live example - http://www.cems.uwe.ac.uk/~r4-george...php?p=products

Everything is in the right directory. When I inspect the 'Add to Cart' button in chrome I get the following -

Uncaught TypeError: Cannot set property 'value' of undefined
addtocartindex.php:51
(anonymous function)index.php:83
onclick
Any help is really appreciated, I'm struggling to see what I have done wrong. I don't know whether it's a Javascript problem.

If you need any of the code from other pages I can post it too.

Thanks in advance.