public function xmlOutput() {
        $o = '';
        $o.= '        <KEY NAME="' . $this->name . '"';
        $o.= ' TYPE="' . $this->getXMLDBKeyName($this->type) . '"';
        $o.= ' FIELDS="' . implode(', ', $this->fields) . '"';
        if ($this->type == XMLDB_KEY_FOREIGN ||
            $this->type == XMLDB_KEY_FOREIGN_UNIQUE) {
            $o.= ' REFTABLE="' . $this->reftable . '"';
            $o.= ' REFFIELDS="' . implode(', ', $this->reffields) . '"';
        }
        if ($this->comment) {
            $o.= ' COMMENT="' . htmlspecialchars($this->comment) . '"';
        }
        $o.= '/>' . "\n";

        return $o;
    }