WordPress previous_post_link exclude posts with multiple categories

I have a single-news page in WordPress where I am looping over all the posts and conditionally updating the previous and next buttons to exclude certain categories based on the current post’s category ID.

Here is what I have:

          <?php if (have_posts()): ?>
            <?php while (have_posts()): ?>
                <?php the_post(); ?>

                <?php if ( in_category(7)) : ?>

                    <?php
                        $sidebar="blog-news";        
                        $catagory = array(3,5,6,4,1);
                    ?>
                <?php endif; ?>

        <nav>
            <ul class="pager">
                <li class="prev">
                    <?php 
                        echo previous_post_link( "%link", "Previous", true, $catagory );
                    ?>
                </li>
                <li class="next">
                    <?php 
                        echo next_post_link( "%link", "Next", true, $catagory );
                    ?>
                </li>
            </ul>
        </nav>
    <?php endwhile; ?>
    <?php endif; ?>

The problem I’m having is that if a post has 2 categories e.g. 6 and 7, it excludes that post from the link, whereas if the post has only one category e.g. 6 then it does not exclude it.

How do I set it so that it DOES NOT exclude any post that has category 7, regardless of if it has multiple categories.

What you need to do is change the excluded_terms argument of the previous_post_link and next_post_link functions which you called $catagory. Simply remove all the category ids of your current post from that excluded_terms array. Make sure to define an array of categories which you don’t want to be displayed like you did it in your example above.

$catagory = array(3,5,6,4,1) // array of category ids you don't want to be displayed

the next step is to find all the category ids of your current post and put it in a new array.

$post_category_objects = get_the_category(); // returns an array of WP_Term objects
$post_category_ids = array();
foreach($post_category_objects as $pco){
     array_push($post_category_ids, $pco->term_id); // adds the post's category id to the $post_category_ids array
}

next you need to remove the category ids of the current post from $catagory (<– the array which contains the values to be excluded from the previous_post_link and next_post_link functions).

Assuming the post has the categories 6 and 7, these category ids will be removed from $catagory.

// removes the current post's category ids from the $catagory-array
   foreach( $post_category_ids as $pci ){
        $key = array_search( $pci, $catagory );
        if( $key !== false ){
             unset( $catagory[$key]);
        }
   }
// rearranges the $catagory's keys
   $catagory = array_values($catagory);

Put the whole code above in your if-statement and it should work.