ldap_search filter with memberOf

I am trying to construct a filter for ldap_search that tests the AD memberOf attribute. The latter has possibly multiple values for a given user, and I want something like (memberOf=*Student*) to get all and only the students. I realize that the above does not work, that the memberOf attribute has some internal structure that requires something more complicated, but I cannot seem to get it working. What I have done is use a very broad $filter, an $attribs that includes "memberOf"-- and then loop through the results keeping only the students, thus: $sr= ldap_search($ldapconn, $dn, $filter, $attribs); for ($entryid=ldap_first_entry($ldapconn,$sr);$entryid!=false;$entryid=ldap_next_entry($ldapconn,$entryid)){ $this_memberof = implode(ldap_get_values($ldapconn,$entryid,'memberof')); If (stripos($this_memberof,'student') ) { // do something, etc. } } But this is wildly inefficient. How can I just create the filter I need for ldap_search? Thanks! Alan