Exclude first 5 posts of specific categories in the main loop

Question

Here is a concept that you can try out to exclude 5 posts per category each from category 3 and 5 where posts are excluded that belongs to category 9 and either of the before mentioned categories.

Here is the function: (CAVEAT: Requires PHP 5.4+ as I used new short array syntax. Change as needed)

/**
 * function get_sticky_category_posts()
 *
 * Get an array of post ids according to arguments given.
 *
 * @author Pieter Goosen
 * @see https://wordpress.stackexchange.com/q/187477/31545
 *
 * @param array $args
 */
function get_sticky_category_posts( $args = [] )
{
    /*
     * If the array is empty or not valid, return null
     */
    if ( empty( $args ) || !is_array( $args ) )
        return null;

    $ids="";

    foreach ( $args as $k=>$v ) {

        /*
         * Check that $v['taxonomy'], $v['included_terms'] and $v['posts_per_page'] are all set. If not, return null
         */ 
        if (    !isset( $v['taxonomy'] ) 
             || !isset( $v['included_terms'] )
             || !isset( $v['posts_per_page'] )
        )
        return null;

        /*
         * Sanitize and validate user input
         */
        $included_terms = filter_var( $v['included_terms'], FILTER_VALIDATE_INT, ['flags'  => FILTER_FORCE_ARRAY] );
        $taxonomy       = filter_var( $v['taxonomy'], FILTER_SANITIZE_STRING );

        /*
         * Create tax_query according to whether $v['excluded_terms'] is set
         */
        if ( !isset( $v['excluded_terms'] ) ) {
            $tax_query = [
                [
                    'taxonomy'         => $taxonomy,
                    'terms'            => $included_terms,
                    'include_children' => false,
                ],
            ];
        } else {
            $tax_query = [
                [
                    'taxonomy'         => $taxonomy,
                    'terms'            => $included_terms,
                    'include_children' => false,
                ],
                [
                    'taxonomy'         => $taxonomy,
                    'terms'            => filter_var( $v['excluded_terms'], FILTER_VALIDATE_INT, ['flags'  => FILTER_FORCE_ARRAY] ),
                    'include_children' => false,
                    'operator'         => 'NOT IN'
                ],
            ];
        } // endif ( !$v['excluded_term'] ) statement

        /*
         * Use get_posts to get an array of post ids to exclude
         */
        $posts_to_exclude = get_posts(
            [
                'posts_per_page' => filter_var( $v['posts_per_page'], FILTER_VALIDATE_INT ),
                'fields'         => 'ids',
                'tax_query'      => $tax_query
            ]
        );
        if ( $posts_to_exclude ) {

            /*
             * Concatenate all ids into a string using implode
             */
            $ids .= implode( ' ', $posts_to_exclude );
            $ids .= ' ';
        } //endif ( $posts_to_exclude )

    } //end foreach

    /*
     * If we don't have any ids, thus empty string, return null
     */
    if ( !$ids )
        return null;

    /*
     * Create a single flat array of post ids. Use rtrim to remove trailing white space
     */
    return explode( ' ', rtrim( $ids ) );
}

Her is how it works:

You need to pass a multi-dimensional array to the function’s arguments in the following format

$args = [
    0 => ['posts_per_page' => 5, 'taxonomy' => 'category', 'included_terms' => 3, 'excluded_terms' => 9],
    1 => ['posts_per_page' => 5, 'taxonomy' => 'category', 'included_terms' => 4, 'excluded_terms' => 9],
];
$a = get_sticky_category_posts( $args );

There are four allowed parameters, of which you have to set the posts_per_page, taxonomy and included_terms (which needs to be an integer or an array of term ids) parameters, otherwise the function returns null. The other allowed parameter (which is not required and can be omitted) is excluded_terms which is also an integer or an array of term ids and is the terms in which a posts should not be in. The above will return the following (ie means the following)
- 5 post ids from the taxonomy term 3 from the taxonomy category, but posts that are in term 3 and 9 should not be returned
- 5 post ids from the taxonomy term 4 from the taxonomy category, but posts that are in term 4 and 9 should not be returned
- The returned array from the function (var_dump( $a )) looks something like this
```
array(10) {
  [0]=>
  string(3) "159"
  [1]=>
  string(3) "149"
  [2]=>
  string(3) "129"
  [3]=>
  string(3) "126"
  [4]=>
  string(3) "119"
  [5]=>
  string(2) "76"
  [6]=>
  string(3) "528"
  [7]=>
  string(3) "394"
  [8]=>
  string(3) "147"
  [9]=>
  string(2) "97"
}
```

This array can now be passed to pre_get_posts to the post__not_in parameter to exclude the posts from the function. You can also use this array of ids in any other place, maybe return them to a custom query or other purposes like sticky posts on other pages except the home page as used in this answer 🙂

Here is a use case with pre_get_posts. This will remove the required posts from the main query. (Requires PHP 5.4+ due to the syntax in the function and the use of closures (closures are available PHP 5.3+)

add_action( 'pre_get_posts', function ( $q )
{
    if (    !is_admin() // Not necessary for home page, included it to be used on any other template like category pages
         && $q->is_main_query() // Make sure this is the main query
         && $q->is_home() // Targets the home page only
    ) {
        if ( function_exists( 'get_sticky_category_posts' ) ) {

            $args = [
                0 => ['posts_per_page' => 5, 'taxonomy' => 'category', 'included_terms' => 3, 'excluded_terms' => 9],
                1 => ['posts_per_page' => 5, 'taxonomy' => 'category', 'included_terms' => 4, 'excluded_terms' => 9],
            ];
            $sticky_cat_posts = get_sticky_category_posts( $args );

            if ( !empty( $sticky_cat_posts ) ) {
                $q->set( 'post__not_in', $sticky_cat_posts );
            }   
        }
    }
});