使用PHP和LDAP列出AD组成员

I am attempting to list group members for groups in a certain OU in Active Directory using PHP and LDAP. For the most part this works but if a user is from another domain, it won't pick up on them. Below is the most basic attempt I have tried. I have tried other code examples as well. With debugging turned on to 7, I see it binding to say the Asia domain and I can see the DN of the user in the error_log but it doesn't return in php.

<?php
//LDAP server address
$LDAP_Server = "ldap://americas.ad.company.com";
//AD User to use
$LDAP_User = "username@americas.ad.company.com";
//AD User Password
$LDAP_Password = "Password";
//FQDN path where search will be performed. OU - organizational unit / DC - domain component
$LDAP_DN = "OU=SITE,OU=US,DC=americas,DC=ad,DC=company,DC=com";
$LDAP_Search_String = "(&(objectCategory=person)(objectClass=user)(memberof=CN=SITE-UG-Group-M,OU=FileServer,OU=Groups,OU=SITE,OU=US,DC=americas,DC=ad,DC=company,DC=com))";
// connecting to LDAP server
$LDAP_Connection = ldap_connect($LDAP_Server);
$LDAP_Bind = ldap_bind($LDAP_Connection, $LDAP_User , $LDAP_Password);
ldap_set_option($LDAP_Connection, LDAP_OPT_PROTOCOL_VERSION, 3);
ldap_set_option($LDAP_Connection, LDAP_OPT_REFERRALS, 0);
// performing search
$LDAP_Search = ldap_search($LDAP_Connection, $LDAP_DN, $LDAP_Search_String);
// Sort the results based on description
ldap_sort($LDAP_Connection, $LDAP_Search, 'samaccountname');

$LDAP_Data = ldap_get_entries($LDAP_Connection, $LDAP_Search);

echo "Found " . $LDAP_Data["count"] . " members<br><br>";

for ($i=0; $i<$LDAP_Data["count"]; $i++) {
    echo $LDAP_Data[$i]["dn"] . "<br>" . $LDAP_Data[$i]["cn"][0] . "<br>";
}
?>