Thread:Extension talk:LDAP Authentication/using LDAP in Global Catalog with a Microsoft forest/reply (2)

I'm not sure that is an option in PHP. According to a comment on php.net, you set the bind address to the server (no ldap://), change the port to 3268, and don't use a base dn, and it'll search the global catalog. I may add an option in a future version to do this.