如何取得MySQL中下一个 Autoindex 的值

<?
$tablename = "tablename";
$next_increment = 0;
$qShowStatus = "SHOW TABLE STATUS LIKE '$tablename'";
$qShowStatusResult = mysql_query($qShowStatus) or die ( "Query failed: " . mysql_error() . "
" . $qShowStatus );

$row = mysql_fetch_assoc($qShowStatusResult);
$next_increment = $row['Auto_increment'];

echo "next increment number: [$next_increment]";
?>