How to display only certain categories in Wordpress - php

Question

I'm working in Wordpress and implemented some PHP to display all categories on a page (by a shortcode). By clicking on a category it links to a new page displaying all posts of that very category.
How do I display only certain categories, for instance by id and/or name of the cateogry?
One post has two categories (a : A & B or A & C). So one post a always has one category A, and one category B or C.
Here's my code:
function swerft_categories( ){
ob_start();
$categories = get_categories();
echo '<div class="swerft_cat">';
foreach($categories as $category) {
echo '<div class="swerft_cat_single col-lg-3 col-md-3 col-sm-6 col-xs-12">';
echo '<div class="swerft_cat_single_inner">';
$thim_group_custom_title_bg_img = get_term_meta( $category->term_id, 'thim_group_custom_title_bg_img', true );
if ($thim_group_custom_title_bg_img) {
$image_id = $thim_group_custom_title_bg_img['id'];
if ($image_id) {
$post_thumbnail_img = wp_get_attachment_image_src( $image_id, 'full' );
echo '<img src="' . $post_thumbnail_img[0] . '" alt="' . $category->name . '" />';
}
}
echo '<h5>'. $category->name .'</h5>';
echo '<p>'. $category->description . $category->count . '<span> Seminare </span>' . '</p>';
echo '</div>';
echo '</div>';
}
echo '</div>';
return ob_get_clean();
}
add_shortcode( 'swerft_categories', 'swerft_categories' );
I tried this in the first few lines for example with no success:
function swerft_categories($args){
ob_start();
$args = array('hide_empty'=> 1,
'name' => 'B');
$categories = get_categories($args);
1) I want only one certain relation to be displayed. Let's say: only the relation of a : A & B
2) I want the count to only show the amount of posts based on the relation above.
3) By clicking on a category based on this relation, I want only those posts to be displayed of course.

Answer 1

I was able to find a solution for the above mentioned problem together with a colleague. See the code below, I hope it may help someone maybe; and thanks for anyone considering this task though ;)
// Function to only show individual seminars after click on category on category-page
function swerft_filter_posts_open_individual_seminare( $query ) {
$offene_seminare = isset($_GET['offene_seminare']) ? boolval($_GET['offene_seminare']) : false;
$individual_seminars_category_id = 91;
if($query->is_category() && $query->is_main_query()) {
if ($offene_seminare) {
$query->set( 'category__not_in', $individual_seminars_category_id );
} else {
$query->set( 'category__in', $individual_seminars_category_id );
}
}
}
add_action( 'pre_get_posts', 'swerft_filter_posts_open_individual_seminare' );
// Function to only count individual seminars in overview
function swerft_count_individual_seminars_in_category($category_id) {
$individual_seminars_category_id = 91;
$query_args = array(
'post_type' => 'post',
'category__and' => array($individual_seminars_category_id, $category_id)
);
$query = new WP_Query( $query_args );
$count = $query->found_posts;
return $count;
}