function _write_header($fd)
	{
		$buf = pack('a3CiiIfa12', XDB_TAGNAME, $this->version,
			$this->hash_base, $this->hash_prime, 0, XDB_FLOAT_CHECK, '');

		fseek($fd, 0, SEEK_SET);
		fwrite($fd, $buf, 32);
	}