What do add_filters() and apply_filter() do?

Question

I'm a bit confused as to why this isn't working - although it must be said I'm not that sure what apply_filters() and add_filter are doing, so any general tips would be great too!

I want a query that brings up the five earlier posts on a single post page. I am sending the current post's date off, and want to apply a filter that filters out posts earlier than this.

function sw_filter_posts_before( $where = '', $date) {

    $where .= " AND post_date < '" . $date . "'";
    return $where;
}

How do I correctly apply this? Simply using add_filter or apply_filter before instantiating a new WP_Query object doesn't seem to work correctly.

Thanks in advance!

Edit: To go into things further, I would like to understand how to pass a variable into the filter, as I can't get $date to pass from another function.

Here is said other function (it is an ajax call within wordpress, hence I start by getting the post ID for the current page through a $_POST variable):

function create_more_videos_sidebar() {

    $id = $_POST['theID'];

    $args = array(  'post_type'         =>  'videos',
                    'posts_per_page'    =>  1,
                    'p'             =>  $id
                ); 

    $wp_query = new WP_Query($args);

    while ($wp_query->have_posts()) : $wp_query->the_post(); $do_not_duplicate = $post->ID;

        $date = get_the_date('Y-m-d');

    endwhile;

    $args = array(  'post_type'         =>  'videos',
                    'posts_per_page'    =>  5
                ); 

    add_filter( 'posts_where', 'sw_filter_videos_before' ); //don't know how to pass $date


    $wp_query = new WP_Query($args);
    remove_filter( 'posts_where', 'sw_filter_videos_before' );

    //do loop stuff

    $response = json_encode( array( 'result' => $result ) );

    header( "Content-Type: application/json" );
    echo $response;

    exit;
}

Answer 1

What are you attempting to filter? I'll assume you're trying to add a filter to a filter hook called posts_before. In which case, you need to add your filter to this hook, via add_filter():

function mytheme_filter_posts_before( $where = '', $date) {

    $where .= " AND post_date < '" . $date . "'";
    return $where;
}
// Add function to the filter hook
add_filter( 'posts_before', 'mytheme_filter_posts_before' );

Note that I changed your function name. filter_posts_before() is far too generic of a function name, and very likely to cause a function-naming collision.

EDIT

And just to clarify:

• apply_filters() is the filter hook location, is called by core code, and is used to apply any filters that are added to the queue by Themes/Plugins (and core).
• add_filter() is called by Themes/Plugins (and core), and is used to add filters to the queue to be applied to the hook by core.

EDIT 2

Based on your comment above, the hook is posts_where. So, let's take a crack at re-building your callback function:

function mytheme_filter_posts_where( $where ) {

    // Here, we need to figure out how to
    // determine what date to use. In your 
    // code example, you call get_the_date(),
    // but this function must be used inside
    // the Loop. Let's try get_the_time()
    // instead. You'll just need to come up
    // with a way to determine what post ID to use.
    $post = 'some_post_id';
    $date = get_the_time( 'Y-m-d', $post );

    $where .= " AND post_date < '" . $date . "'";
    return $where;
}
// Add function to the filter hook
add_filter( 'posts_where', 'mytheme_filter_posts_where' );