Using one category page to show multiple categories in WordPress

[Update: There is probably never a good reason to do this. Instead, create a new category to hold the posts.]

Trying to show multiple categories in one loop is easily the hardest thing I’ve done in WordPress.

First, create a container category where you want your multiple categories to be shown. Let’s call it the MultiCat category and give it the multicat slug. No posts are required to belong to this category, and if they do, it will have no benefit.
Next, add this bit of code to functions.php of your theme. This is where we create a query variable which enables us to identify the multi-category page properly. Update the category slugs below to match the slugs of the categories you want to show together.
```
function multi_cat_handler( $query ) {
    if ( $query->is_main_query() && $query->query["category_name"] == 'cat1-slug,cat2-slug,cat3-slug,cat4-slug' ) {
     $query->set("allish",'yes');
    }
}
add_action( 'pre_get_posts', 'multi_cat_handler' );
```

Next, add this code to functions.php. Update multicat to the slug of your multiple categories category. Also update the other slugs as in the previous step.

function alter_the_query_for_me( $request ) {
    $dummy_query = new WP_Query(); 
    $dummy_query->parse_query( $request );
	  if($dummy_query->query['category_name'] == 'multicat') {
		$request['category_name'] = 'cat1-slug,cat2-slug,cat3-slug,cat4-slug';
	  }
    return $request;
}
add_filter( 'request', 'alter_the_query_for_me' );

To display the h1 tag of the MultiCat category page properly, we use the following code:
```
if(get_query_var('allish') == 'yes') {
echo 'Title of the Multiple Categories Page';
}
else {
echo 'Normal code that outputs category title';
}
```
If you do not do the above, when people go to the MultiCat category page, they will see a random title from one of the multiple categories you want to show on the page, which is not the behavior you want.
Below is the main code that outputs your posts. The if clause at the top allows us to know we are on the multiple categories page (we cannot use other methods such as checking category ID, since that will return a random category’s ID from the multiple categories we want to show).
```
Here lies the code that outputs your post content





Here is the loop that outputs your normal categories
```
The $args array contains the query we use to pull posts from the database. We are pulling posts from the categories with the IDs of 3, 4, 671 and 672. Notice that in Step 2 we used category slugs, while in this step we are using category IDs. They have to match, and order may matter.

That’s all.

Caveats

The RSS feed of the category page will be the RSS feed of one of the categories shown on the MultiCat page. This may be fixable through using RSS-specific filters, but in my case I had no need for RSS and did not try to find a fix.