Customizing the WordPress post_class Filter

post_class_filter.php

<?php

function my_post_classes( $classes ) {
    global $multipage, $page, $post, $wp_taxonomies;
        
    if($wp_taxonomies) {
        foreach($wp_taxonomies as $taxonomy) {    
            $tax_array[] = $taxonomy->name;
        }
        
        $taxes = wp_get_object_terms(get_the_id(), $tax_array, array('fields' => 'all'));

        if($taxes) {
            foreach($taxes as $k => $v) {
                $classes[] = $v->taxonomy .  '_' . $v->slug;
           }
        }
    }

    // if it's a paged post
    if ( $multipage ) {
        $classes[] = 'paged-post';
        $classes[] = 'post-page-'.$page;
    }

    // if it's the first page of the post
    if ( 1 === $page ) {
        $classes[] = 'post-first-page';
    }

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

add_filter( 'post_class', 'my_post_classes' );