Add class name for top-level parent category or page to body tag in WordPress posts, pages, and archives.

wp-top-parent-body-class.php

<?php
/**
 * Insert class into body tag for highest level Category or Page.
 *
 * Pages and Categories with identical slugs get the same class.
 * Works on Pages, Posts, and Category achives.
 *
 * @return string $top_slug  Class name for body tag.
 */
function top_cat_or_page_body_class( $class ) {
    $prefix = 'topic-'; // Editable class name prefix.
    global $wp_query;
    $object_id = $wp_query->get_queried_object_id();
    $top_slug  = ( is_home() ) ? 'home' : 'default';

    if ( is_single() ) {
        $cats    = get_the_category( $object_id ); // Get post categories.
        $parents = get_ancestors( $cats[0]->term_id, 'category', 'taxonomy' );
        $top_id  = ( $parents ) ? end( $parents ) : $object_id;

        // If term has parents, get ID of top page.
        $top_cat  = get_category( $top_id ); // Get top cat object.
        $top_slug = $top_cat->slug; // Get top cat slug.
    }

    if ( is_category() ) {
        $parents = get_ancestors( $object_id, 'category', 'taxonomy' );
        $top_id  = ( $parents ) ? end( $parents ) : $object_id;

        // If cat has parents, get ID of top cat.
        $top_cat  = get_category( $top_id ); // Get top cat object.
        $top_slug = $top_cat->slug; // Get top cat slug.
    }

    if ( is_page() ) {
        $parents = get_ancestors( $object_id, 'page', 'post_type' );
        $top_id  = ( $parents ) ? end( $parents ) : $object_id;

        // If page has parents, get ID of top page.
        $top_page = get_post( $top_id ); // Get top page object.
        $top_slug = $top_page->post_name; // Get top page slug.
    }

    $class[] = $prefix . $top_slug;

    return $class;
}
add_filter( 'body_class', 'top_cat_or_page_body_class' );

/*
// Rewrote the above in 2021. Below is the legacy function:

// Add body-class for top-level parent Page or Category
function topcatpg_body_class( $class ) {
	$prefix = 'topic-'; // Editable class name prefix.
	$top_cat_pg = 'home'; // Default.
	global $top_cat_pg;

	// Get class name from top-level Category or Page.
	global $wp_query;
	if ( is_single() ) {
		$wp_query->post = $wp_query->posts[0];
		setup_postdata( $wp_query->post );
		 // Climb Posts category hierarchy, successively replacing
		 // class name top_cat_pg with slug of higher level cat.
		foreach( (array) get_the_category() as $cat ) {
			if ( !empty( $cat->slug ) )
				$top_cat_pg = sanitize_html_class( $cat->slug, $cat->cat_ID );
			while ( $cat->parent ) {
				$cat = get_category( (int) $cat->parent);
				if ( !empty( $cat->slug ) )
					$top_cat_pg = sanitize_html_class( $cat->slug, $cat->cat_ID );
			}
		}
	} elseif ( is_archive() ) {
		if ( is_category() ) {
			$cat        = $wp_query->get_queried_object();
			$top_cat_pg = $cat->slug;
			// Climb Category hierarchy, successively replacing
		 	// class name with slug of higher level cat.
			while ( $cat->parent ) {
				$cat = get_category( (int) $cat->parent );
				if ( !empty( $cat->slug ) )
					$top_cat_pg = sanitize_html_class( $cat->slug, $cat->cat_ID );
			}
		}
	} elseif ( is_page() ) {
		global $post;
		if ( $post->post_parent )	{
			$ancestors  = get_post_ancestors( $post->ID );
			$root       = count( $ancestors ) - 1;
			$top_id     = $ancestors[$root];
			$top_pg     = get_page( $top_id );
			$top_cat_pg = $top_pg->post_name;
		} else {
			$top_cat_pg = $post->post_name;
		}
	}
	$class[] = $prefix . $top_cat_pg;

	return $class;
}
// Uncomment to use this function instead ot the above newer one:
// add_filter( 'body_class', 'topcatpg_body_class' );
*/
?>

This code works SO well, and I really want to keep using it, but I seem to be getting daily errors in my php_errorlog:

[20-Feb-2023 20:17:03 UTC] PHP Warning: Attempt to read property "slug" on null in /home/customer/www/[website folder]/public_html/wp-content/themes/generatepress_child/functions.php on line 91

It's referring to the first instance of $top_slug = $top_cat->slug; // Get top cat slug.

Is there something I can do prevent the errors? The code is working perfectly otherwise!

@sambieluy
None of the sites I work with are using this script now. So I'm just guessing, not testing, but maybe I need to wrap that error-producing line in an if, to trap for get_category( $top_id ) not returning a category (because, if it did, the cat would have a slug), like:
$top_slug = ($top_cat) ? $top_cat->slug : $top_slug;

If you have a chance to test that, please do. Tell me if it gets rid of the errors.

Thanks so much for your response, @hearvox :)

I tried changing $top_slug = $top_cat->slug to if ( $top_slug = $top_cat->slug ); yesterday (Feb. 21st), and it seems like 3 more of the same errors came through today. Interestingly, there are way fewer errors this time, but I'm not sure if that's relevant.

[22-Feb-2023 16:42:41 UTC] PHP Warning: Attempt to read property "slug" on null in /home/customer/www/[website folder]/public_html/wp-content/themes/generatepress_child/functions.php on line 91
[22-Feb-2023 16:45:23 UTC] PHP Warning: Attempt to read property "slug" on null in /home/customer/www/[website folder]/public_html/wp-content/themes/generatepress_child/functions.php on line 91
[22-Feb-2023 18:40:26 UTC] PHP Warning: Attempt to read property "slug" on null in /home/customer/www/[website folder]/public_html/wp-content/themes/generatepress_child/functions.php on line 91

Could it be because I implemented the "if" statement incorrectly (screenshot attached for more context)? Do I need to have some kind of dependent condition afterwards? Really sorry for my lack of knowledge with PHP!

@hearvox Just a quick update that we're back to the same number of "Attempt to read property 'slug' on null" errors today (16 instances so far today), so ignore what I said about there being fewer errors. It must be based on how many users cause the code to run.