Intelligent Post Terms - use most appropriate category for WordPress post links.

intelligent-post-terms.php

<?php

/**
 * Intelligent Post Terms
 *
 * If the post category is included in the permalink for posts
 * we can be clever and detect when a post is displayed or linked to
 * which is the most suitable category to use in the URL.
 *
 * @version  1.0
 */

add_filter( 'post_link_category', array( 'Intelligent_Post_Terms', 'post_link_category' ), 10, 3 );
add_filter( 'get_the_categories', array( 'Intelligent_Post_Terms', 'get_the_categories' ) );

class Intelligent_Post_Terms {

	/**
	 * Post Link Category
	 *
	 * Gets categories associated with a post an returns
	 * the most appropriate category for the context.
	 *
	 * @param   WP_Term  $cat   Category object.
	 * @param   array    $cats  Post category objects.
	 * @param   WP_Post  $post  Post.
	 * @return  WP_Term         Category object.
	 */
	public static function post_link_category( $cat, $cats, $post ) {

		$category_name = self::get_query_category_name();

		if ( ! empty( $category_name ) ) {
			$category = wp_list_filter( $cats, array( 'slug' => $category_name ) );
			if ( ! empty( $category ) ) {
				return current( $category );
			}
		}

		return $cat;

	}

	/**
	 * Get the Categories
	 *
	 * @param   array  $categories  Category objects.
	 * @return  array               Category objects.
	 */
	public static function get_the_categories( $categories ) {

		$category_name = self::get_query_category_name();

		if ( ! empty( $category_name ) ) {
			$category = wp_list_filter( $categories, array( 'slug' => $category_name ) );
			if ( ! empty( $category ) ) {
				$categories = wp_list_filter( $categories, array( 'slug' => $category_name ), 'NOT' );
				$categories = array_merge( $category, $categories );
			}
		}

		return $categories;

	}

	/**
	 * Get Query Category Name
	 *
	 * Gets the 'category_name' query var and if it's a child category
	 * remove the category ancestors to get the category slug.
	 *
	 * @return  string  The category slug.
	 */
	public static function get_query_category_name() {

		$category_name = get_query_var( 'category_name' );

		// If category name is a path, strip category ancestors
		if ( ! empty( $category_name ) && false !== strpos( $category_name, '/' ) ) {
			$category_name_parts = explode( '/', $category_name );
			$category_name = end( $category_name_parts );
		}

		return $category_name;

	}

}