OD and ldap_search syntax problem...

I am trying to create some php pages which create on the fly directory pages for our users (images, email links, vcards etc) - all from the ldap server - i want to select users by group membership and have been trying to use ldap_search i have been creating a filter that i think should be as follows:-

$filter = "(&(objectClass=InetOrgPerson)(memberOf=cn=somegroup,cn=groups,dc=domain,dc=org ))";

this does not work.. i get a result with all inetorgpersons if i leave off the memberof statement

$filter = "(objectClass=InetOrgPerson)";

it is the same if i connect with a bind and password or anonymous...