Description:
------------
When using HTML checkboxes in an application, the checkbox is not recognized in HTTP_POST_VARS unless it is checked.  If the checkbox is checked, the value of "customer_active" is "on", however it the checkbox is not checked, no value exists and HTTP_POST_VARS does not recognize the entity "customer_active".

I believe HTTP_POST_VARS should always recognize a defined HTML entity regardless of its state (i.e., checked or unchecked)

Reproduce code:
---------------
if ($m['customer_active'] == 'yes') {$customer_active = 'CHECKED';
} else {
$customer_active = '';
};

print '<form name=EditCustomer action="submit_customer.php" method="post" enctype="multipart/form-data" onsubmit="return CheckForm(this)">';

print '<tr>'; 
print '<td class=fieldname>Activate Customer:</td>';
print '<td class=formfield><input type="checkbox" name="customer_active" '.$customer_active.' ></td>';
print '</tr>';

print '<TR>'; 
print '<TD width="20%" height="19"><INPUT class=FormAdmin type="submit" value="Submit" >&nbsp;&nbsp;<button class=FormAdmin type="button" onclick="history.back()">Cancel</button></TD>';
print '<TD width="80%" height="19"></TD>';
print '</TR>';

Expected result:
----------------
If the checkbox is unchecked, the value of "customer_active" should be "off" instead of HTTP_POST_VARS reporting that it is undefined.

Actual result:
--------------
If the checkbox "customer_active" is checked, then HTTP_POST_VARS['customer_active'] has a value of 'on'.  If it is unchecked, there is no defined HTTP_POST_VARS['customer_active'].