Doing a tax-included/excluded override based on customers group

I'm trying to do a simple override, at least I thought so, to hide tax from prices for resellers.

Tried adding a new file to 'includes/extra_configures', 'includes/extra_datafiles' etc. but it seems like $_SESSION['customer_id'] isn't loaded at this point? Wonder where I went wrong, and if anyone can get me back on track.

<?php

// get customers group
if($_SESSION['customer_id']) {
  global $db, $customers_group;
     $customer_group_query = "select gp.group_name
                               from " . TABLE_CUSTOMERS . " cu
                               left join " . TABLE_GROUP_PRICING . " gp on cu.customers_group_pricing=gp.group_id
                               where cu.customers_id = " . $_SESSION['customer_id'];
      if($customer_group = $db->Execute($customer_group_query)) {
        $customers_group=$customer_group->fields['group_name'];
      }
}

// if reseller display prices ex tax
if ($customers_group == 'Reseller') {
    define('DISPLAY_PRICE_WITH_TAX', 'false');
}

Two problems:

1. The session isn't started yet
2. By the time the session is started, you won't be able to define DISPLAY_PRICE_WITH_TAX to a different value, because it will already be defined

Thanks for your reply DrByte, I appreciate it.

If I only have define('DISPLAY_PRICE_WITH_TAX', 'false'); in the file it will display all prices ex tax, but then it's for all - and of no use.

I was hoping I wouldn't need to change lot's of core files to make this work, as the tax handling isn't fully centralized - seems like the define is used in 32 lines (15 files) in catalog part (+ plugins).

Does anyone see any possible workaround for this, without requiring full rewrite/override of tax function and/or all those files?

When the store is operating in "DISPLAY_PRICE_WITH_TAX = 'false'" mode, then it's easier to skip taxes because they're basically assessed by an order-total module, which can be disabled selectively.

But when you're using DISPLAY_PRICE_WITH_TAX='true' mode, it's definitely more complicated.
I suppose you could set a session cookie during login and put your earlier code (after changing it to read the cookie instead of the session) into an init script set to load at an autoloader point between 30 and 40 ... say maybe 37. When the cookie is set during login and the customer is redirected back to (whatever page in) the store, it could then read the cookie and take action accordingly.
I'm not a big fan of using cookies for anything more than absolutely necessary, but technically it would probably accomplish your goal.

OK, don't want to go into that cookie thing - so I just hacked the core files.

Added to end of application_top.php

// get customers group
if($_SESSION['customer_id']) {
  global $customers_group;
     $customer_group_query = "select gp.group_name
                               from " . TABLE_CUSTOMERS . " cu
                               left join " . TABLE_GROUP_PRICING . " gp on cu.customers_group_pricing=gp.group_id
                               where cu.customers_id = " . $_SESSION['customer_id'];
      if($customer_group = $db->Execute($customer_group_query)) {
        $customers_group=$customer_group->fields['group_name'];
      }
}

And then changed all

DISPLAY_PRICE_WITH_TAX == 'true'

into

DISPLAY_PRICE_WITH_TAX == 'true' && $customers_group != 'Reseller'