public static function fetch($ldap, $dn = null)
    {
        if (!$ldap instanceof Net_LDAP2) {
            return PEAR::raiseError("Unable to fetch Schema: Parameter \$ldap must be a Net_LDAP2 object!");
        }

        $schema_o = new Net_LDAP2_Schema();

        if (is_null($dn)) {
            // get the subschema entry via root dse
            $dse = $ldap->rootDSE(array('subschemaSubentry'));
            if (false == Net_LDAP2::isError($dse)) {
                $base = $dse->getValue('subschemaSubentry', 'single');
                if (!Net_LDAP2::isError($base)) {
                    $dn = $base;
                }
            }
        }

        // Support for buggy LDAP servers (e.g. Siemens DirX 6.x) that incorrectly
        // call this entry subSchemaSubentry instead of subschemaSubentry.
        // Note the correct case/spelling as per RFC 2251.
        if (is_null($dn)) {
            // get the subschema entry via root dse
            $dse = $ldap->rootDSE(array('subSchemaSubentry'));
            if (false == Net_LDAP2::isError($dse)) {
                $base = $dse->getValue('subSchemaSubentry', 'single');
                if (!Net_LDAP2::isError($base)) {
                    $dn = $base;
                }
            }
        }

        // Final fallback case where there is no subschemaSubentry attribute
        // in the root DSE (this is a bug for an LDAP v3 server so report this
        // to your LDAP vendor if you get this far).
        if (is_null($dn)) {
            $dn = 'cn=Subschema';
        }

        // fetch the subschema entry
        $result = $ldap->search($dn, '(objectClass=*)',
                                array('attributes' => array_values($schema_o->types),
                                        'scope' => 'base'));
        if (Net_LDAP2::isError($result)) {
            return PEAR::raiseError('Could not fetch Subschema entry: '.$result->getMessage());
        }

        $entry = $result->shiftEntry();
        if (!$entry instanceof Net_LDAP2_Entry) {
            if ($entry instanceof Net_LDAP2_Error) {
                return PEAR::raiseError('Could not fetch Subschema entry: '.$entry->getMessage());
            } else {
                return PEAR::raiseError('Could not fetch Subschema entry (search returned '.$result->count().' entries. Check parameter \'basedn\')');
            }
        }

        $schema_o->parse($entry);
        return $schema_o;
    }