See http://pear.php.net/manual/en/package.networking.net-ldap2.connecting.php
scope: Default search scope.
See http://pear.php.net/manual/en/package.networking.net-ldap2.connecting.php
+schema_cachefile: File location to store ldap schema.
+schema_maxage: TTL for cache file.
attributes: an array that relates StatusNet user attributes to LDAP ones
username*: LDAP attribute value entered when authenticating to StatusNet