[WP] Add the page/post categories in the body classes

wp-add-categories-in-body-classes.php

<?php

/**
 * Add the post/page categories in the body class list.
 *
 * @param string[] $classes An array of body class names.
 *
 * @return array
 */
function timwp_add_category_name_to_body_classes( array $classes ): array {
	$post = get_post();
	foreach ( ( get_the_category( $post->ID ) ) as $category ) {
		// add category slug to the $classes array
		$classes[] = 'cat-' . $category->category_nicename;
		$classes[] = 'cat-id-' . $category->term_id;
	}

	// return the $classes array
	return $classes;
}

add_filter( 'body_class', 'timwp_add_category_name_to_body_classes' );