query post limits

Question

function posts_limit_fn($limit, $query, $custom_limit = ''){
    if($custom_limit !== ''){
        return $custom_limit;
    }
    return $limit;
}

this piece of code is executed into an AJAX function add_action('wp_ajax_search_query', 'search_query');

$queryString = array( 's' => $searchTerm, 'cat' => $categoriesIDs, 'post__not_in' => array($excludedPost));

if($queryString != ''){
    add_filter('post_limits', 'posts_limit_fn',10, 3);
    $custom_limit = 'LIMIT 0,4';
    apply_filters('post_limits', '', '', $custom_limit);

    $query = new WP_Query($queryString);
}

For some kinda of reasons the apply_filters doesn't work so the query is returning all posts. What would be the problem ?

UPDATE:

Seems that If I have something like this :

function posts_limit_fn($limit,$query,$custom_limit = ''){
    echo $custom_limit.">>>>>>>";
    if($custom_limit !== ''){
        return 'LIMIT 0,3';
    }
    return $limit;
}

the output is LIMIT 0,4>>>>>>>>>LIMIT 0,10>>>>>>>>>post1,post2,post3

Answer 1

The third parameter for add_filter is the priority in which the filter is executed, the forth is the number of parameters the function in the second parameter uses.

The wpcodex_filter_main_search_post_limits function has an if that checks uses a couple of wordpress function to establish where in your page you are executing the code, in this case it's checking if it's in the frontend, if it's the main query and if it's in the search page.

The parameter $limit and $query are passed by the CMS, and those are default values, so if the conditions in the if aren't met, the function return the default $limit

edit:

function posts_limit_fn($limit, $query){
    return 'LIMIT 0, 3';
}
add_filter('posts_limit', 'posts_limit_fn',10, 2);

edit 2:

if you aren't going to use the parameters you can use the function like this:

global $custom_limit;
$custom_limit = '0, 2';
    function posts_limit_fn(){
            global $custom_limit
            return 'LIMIT ' . $custom_limit;
        }
add_filter('posts_limit', 'posts_limit_fn',10, 0);

That's one way to do it. Try to prefix all global vars, always, and if you can avoid them, better yet.

edit: without globals

function posts_limit_fn($limit, $query, $custom_limit = ''){
                if($custom_limit !== ''){
                    return $custom_limit;
                }
                return $limit;
            }
    add_filter('posts_limit', 'posts_limit_fn',10, 3);
$custom_limit = 'LIMIT 0,4';

apply_filters('posts_limit', '', '', $custom_limit)

edit for WP_Query:

If you are using WP_Query, for the pagination you can pass a paged parameter with each call to the function.