news mod - searches : how can i add a condition to the query ?

Hello, i'm working on a news mod.
Actually, i've add a checkbox in the search page to ask if the user want to search everywhere or in the news only.

<label class="conl"><?php echo $lang_search['Search news'] ?><br />
<input name="isnews" type="checkbox" value="1"><br /></label>

For the news, i added a news field in the punBB_topics table; and the value can be 0 (if it's not a news) or 1.

So i would like to modify the queries in search.php to set this condition :

if ($isnews == "1") {
>>>>> add condition to the query : WHERE news = '1'
}

what have i to modify in search.php to set this condition ?

Thanks a lot !