Starts with wrong domain host name

[meshgraphics]

////////// snip from application_top.php

/**
 * if main configure file doesn't contain valid info (ie: is dummy or doesn't match filestructure, display assistance page to suggest running the installer)
 */
if (!defined('DIR_FS_CATALOG') || !is_dir(DIR_FS_CATALOG.'/includes/classes')) {
  require('includes/templates/template_default/templates/tpl_zc_install_suggested_default.php');
  exit;
}

/**
 * 
Check requested domain name matches Zen Cart config.  Compliments of meshgraphics.com.
 */

if (!defined('HTTP_SERVER') || 'http://'. $_SERVER['SERVER_NAME'] != HTTP_SERVER){
  require('includes/templates/template_default/templates/tpl_zc_install_suggested_default.php');
  exit;
}

[kuroi]

If by mistake you had the same DocumentRoot for both websites, asking for either website would start the same index.php.

In fact, this may be what you want. If you want multiple domain names starting up the same site, this is what you would do.

Exactly right. Different domain names coming to the same front controller (index.php) may be a mistake, though it's more likely to be deliberate. For example I have clients who run two or more sites from the same Zen Cart. Some run multiple retail sites using the same products, one runs a retail and a wholesale site with a common sign on but different products and pricing depending on which domain the customer comes in on (and whether they are authorised for the wholesale site).

Ok, lets suppose Zen Cart is going to check the servername after the php starts up. Apparently, is doesnt do that.

Also correct. If you choose to direct multiple domains at a single front controller, Zen Cart has to assume that you did it deliberately, so there's no corrective or specific action for it to take, and so no reason to perform the check.