LDAP PHP Authentication across multiple AD Domains - php

Question

Firstly, Id like to state that my PHP/LDAP skillset is minimal so am looking for a sudo genius to help me solve this issue.
Having recently merged with another company, the business wants us to allow our users to authenticate against a PHP web application in another Active Directory domain that we have a forest trust with.
Below is the code I've been given for the LDAP connection and the previous person who was a PHP genius has now left the company and this has been lumped onto me to work out. The code is however not working. The users in domain where the PHP application is store are able to access correctly.
<?php
$browser_shortname = explode('\\', $_SERVER['AUTH_USER']);
// Generate the global LDAP connection to the specificed primary server.
$ldap_connection = ldap_connect($ldap_protocol.$ldap_primaryhost) or die( header('Location: /error/?e=LD01'));
ldap_set_option($ldap_connection, LDAP_OPT_PROTOCOL_VERSION, $ldap_protocolversion);
ldap_set_option($ldap_connection, LDAP_OPT_REFERRALS, $ldap_referrals);
if ($ldap_tls == 1) {
ldap_start_tls($ldap_connection);
}
if ($ldap_debug == 1) {
ldap_set_option($ldap_connection, LDAP_OPT_DEBUG_LEVEL, 7);
}
$ldap_binding = ldap_bind($ldap_connection, $ldap_domain.'\\'.$ldap_username, $ldap_password);
if (!$ldap_binding) {
include ('/core/styles/'.$theme_selected.'/templates/101.tpl');
die();
}
$ldapus_filter = "(sAMAccountName=$browser_shortname[1])";
$ldapus_result = ldap_search($ldap_connection, $ldap_dn, $ldapus_filter);
$ldapus_details = ldap_get_entries($ldap_connection, $ldapus_result);
if ($browser_shortname[0] == "MY-DOMAIN") {
$ldap_khaconnection = ldap_connect($ldap_protocol.$ldap_khahost) or die( header('Location: /error/?e=LD01'));
ldap_set_option($ldap_khaconnection, LDAP_OPT_PROTOCOL_VERSION, $ldap_protocolversion);
ldap_set_option($ldap_khaconnection, LDAP_OPT_REFERRALS, $ldap_referrals);
$ldapus_filter = "(sAMAccountName=$browser_shortname[1])";
$ldapus_result = ldap_search($ldap_khaconnection, $ldap_khadn, $ldapus_filter);
$ldapus_details = ldap_get_entries($ldap_khaconnection, $ldapus_result);
echo "This confirms the user is coming from KHA.";
echo $browser_shortname[1];
}
$ldap_userfullname = $ldapus_details[0]["displayname"][0];
$ldap_userfirstname = $ldapus_details[0]["givenname"][0];
$ldap_usertitle = $ldapus_details[0]["title"][0];
$ldap_accountname = $ldapus_details[0]["samaccountname"][0];
?>