Unless you want to do a lot of work yourself, use Google to do the work for you, assuming Google has indexed your site. Example:
<?
$keyword = "nokia 6680";
?>
<form action="http://www.google.com/search">
<input type="hidden" name="hl" value="en">
<input name="q" value="<? echo "site:www.yoursite.com " . $keyword ?>">
<input type="submit" value="Google Search" name="btnG">
</form>
Or just link to something like
http://www.google.com/search?hl=en&q=site%3Awww.yoursite.com+nokia+6880&btnG=Google+Search