<?php

/**
 * All Category Post Permalink
 *
 * When using %category% in permalinks, this allows you to substitute
 * an alternative slug "all" to replace the category. By default this
 * is not permitted - the post page is force redirected to a URL
 * with a valid category.
 */
class All_Category_Post_Permalink {

	/**
	 * Constructor
	 */
	public function __construct() {

		add_filter( 'post_link_category', array( $this, 'post_link_category' ), 3, 3 );

	}

	/**
	 * Post Link Category
	 *
	 * @param   WP_Term  $cat   Category object.
	 * @param   array    $cats  Post categories.
	 * @param   WP_Post  $post  Post object.
	 * @return  string          Replacement slug for %category% placeholder.
	 */
	public function post_link_category( $cat, $cats, $post ) {

		if ( is_main_query() && ( ( is_home() ) || ( is_single() && $this->get_slug() == get_query_var( 'category_name' ) ) ) ) {
			$cat->slug = $this->get_slug();
		}

		return $cat;

	}

	/**
	 * Get "all" Slug
	 *
	 * @return  string  Slug to replace the %category% placeholder if no category.
	 */
	private function get_slug() {

		return sanitize_html_class( apply_filters( 'all_category_post_permalink_slug', 'all' ) );

	}

}

new All_Category_Post_Permalink();