Highlight posts that belong to current sub category

Question

I'm trying to highlight list items that belong to the same category as that of the current category page.

For an example, I have a category called food, with the subcategories: breakfast, lunch and dinner. If you navigate to the sub-category lunch – in a loop that calls all posts regardless of category – I want all list items that are associated to lunch, to be highlighted.

Firstly I use get_category and ->term_id to define the ID of the category we're currently on. Secondly, in the loop, I use get_the_category and [0]->cat_ID to retrieve the ID (but only the first in the array) of each individual list item. Lastly, I use in_array to see if there is a match between the two arguments, and if so echo a value.

My problems lies in the $categories[0]->cat_ID. If a list item is associated to more than one category, retrieving only the first one in the array is no good.

How do I create an argument that will output all category IDs associated with a list item in a loop?

<?php
$thisTrueCat = get_category( get_query_var( 'cat' ) ); 
?>


<!-- loop start -->

<?php 
$categories = get_the_category(); 
$categories_id = $categories[0]->cat_ID;            
?>


<?php if( in_array( $thisTrueCat->term_id, array($categories_id) ) ) { echo 'cat-hi'; }?> 

Answer 1

Instead of:

$categories = get_the_category(); 
$categories_id = $categories[0]->cat_ID;    

...use:

$categories = wp_list_pluck( get_the_category(), 'term_id' );

Now you can correctly use in_array to search all the post's categories, rather than just the first:

if ( in_array( $thisTrueCat->term_id, $categories ) )  { ... }