For anyone interested in removing a user from a group, you can use ldap_mod_del() as follows:
$group = 'CN=mygroup,OU=myOU,DC=mydomain,DC=com';
$group_info['member'] = 'CN=User\, Test,CN=Users,DC=mydomain,DC=com';
ldap_mod_del($ldap, $group, $group_info);
I have tested this using Active Directory on a Win 2K3 server.