A search filter function, that I can't quite make…

Hi,

I am trying to make a function in my functions.php file. Its supposed to loop through search results, find out if they are using a certain category, if they are add them to an array, exclude all id’s in that array from showing in search results. Here’s what I have, I just don’t understand much about this….

function filter_where($where = '') {
	global $wpdb;//is this needed?
	if ( is_search() ) {
		if(is_page_template('landing.php')) { echo 'yes!'; } // this is where I will add id's to the array.
		$exclude = array(286);	

		for($x=0;$x<count($exclude);$x++){
		  $where .= " AND ID != ".$exclude[$x];
		}
	}
	return $where;
}
add_filter('posts_where', 'filter_where');

I have never done anything like this and would like to learn how to use the filters properly…Thanks