LDAP and AD

Hi all I have PHP working on our intranet (linux) and I am trying to get ldap to authenticate against AD using the samaccountname (username) instead of the users cn, which is either the username or full name depending on how the user was created. Authentication against the users' cn is fine but I am stuck on how to do it the the username. Has anyone come across and solved this problem with php or any other ldap authentication TIA matt