WordPress: Post Permalink using primary category

class-primary-category-post-url.php

<?php
/**
 * Alters the default post permalink structure to be:
 *
 * `/{primary-category}/{post-slug}/`
 */

/**
 * The Primary_Category_Post_URL class.
 */
class Primary_Category_Post_URL {

	/**
	 * The permalink structure.
	 *
	 * @var string
	 */
	public $permalink_structure = '/%primary_category%/%postname%/';
  
	/**
	 * Existing instance.
	 *
	 * @var array
	 */
	protected static $instance;
	
	/**
	 * Get class instance.
	 *
	 * @return object
	 */
	public static function instance() {
		if ( ! isset( static::$instance ) ) {
			static::$instance = new static();
			static::$instance->setup();
		}
		return static::$instance;
	}

	/**
	 * Setup the singleton.
	 */
	public function setup() {
		// Set the main permalink structures.
		if ( function_exists( 'wpcom_vip_load_permastruct' ) ) {
			wpcom_vip_load_permastruct( $this->permalink_structure );
		} else {
			add_filter( 'init', function() {
				global $wp_rewrite;
				$wp_rewrite->set_permalink_structure( $this->permalink_structure );
			} );
		}

		// Add rewrite tags.
		add_action( 'init', [ $this, 'add_rewrite_tags' ] );

		// Update post permalinks.
		add_filter( 'post_link', [ $this, 'custom_permalink' ], 10, 2 );
		add_filter( 'post_type_link', [ $this, 'custom_permalink' ], 10, 2 );

		// Filter the post rewrite rules to account for our custom permalink.
		add_filter( 'post_rewrite_rules', [ $this, 'generate_post_rewrite_rules' ] );

		// Add the primary category field.
		add_action( 'fm_post_post', [ $this, 'add_fields' ] );
	}

	/**
	 * Add the custom rewrite tags.
	 */
	public function add_rewrite_tags() {
		add_rewrite_tag( '%primary_category%', '([^/]+)' );
	}

	/**
	 * Replace the wildcard url structure with the correct content
	 *
	 * @param string   $url  The current URL.
	 * @param \WP_Post $post The post object.
	 * @return string $url The new URL.
	 */
	public function custom_permalink( $url, $post ) {
		/**
		 * Filter the array of applicable post types.
		 *
		 * @param array $post_types The array of applicable post types.
		 * @return array $post_types The array of applicable post types.
		 */
		$applicable_post_types = apply_filters(
			'custom_permalink_applicable_post_types',
			[ 'post' ]
		);

		if ( $post instanceof \WP_Post && in_array( $post->post_type, $applicable_post_types, true ) ) {
			$primary_category_id = get_post_meta( $post->ID, 'primary_category', true );

			if ( ! empty( $primary_category_id ) ) {
				$primary_category = get_term( $primary_category_id );

				if ( ! empty( $primary_category->slug ) ) {
					$url = str_replace( '%primary_category%', $primary_category->slug, $url );
				}
			}
		}

		return $url;
	}

	/**
	 * Since we are using a custom permalink structure for this site we wanted to
	 * make sure that its value was correct at all times, which is why we use
	 * `wpcom_vip_load_permastruct( '/%primary_category%/%postname%/' )` in the
	 * `setup()` function.  This will then create new rewrite rules where walk_dirs
	 * is true which is something that we do not want to do.
	 *
	 * This function will add the correct post rewrite rules with walk_dirs false,
	 * and filter only runs when the rewrite rules are created or on `flush_rewrite_rules()`.
	 *
	 * @param \WP_Rewrite $post_rewrite Current WP_Rewrite instance, passed by reference.
	 */
	public function generate_post_rewrite_rules( $post_rewrite ) {
		global $wp_rewrite;
		return $wp_rewrite->generate_rewrite_rules( $this->permalink_structure, EP_PERMALINK, true, true, false, false );
	}

	/**
	 * Add the Primary Category FM field.
	 */
	public function add_fields() {
		$fm = new \Fieldmanager_Select( [
			'attributes' => array(
				'style' => 'width: 95%',
			),
			'remove_default_meta_boxes' => false,
			'name' => 'primary_category',
			'limit' => 1,
			'first_empty' => true,
			'description' => __( '(Required) Used as the first level of the post URL.' ),
			'validation_rules' => 'required',
			'validation_messages' => __( 'Please select a Primary Category' ),
			'datasource' => new \Fieldmanager_Datasource_Term( [
				'taxonomy' => 'category',
				'only_save_to_taxonomy' => false,
				'append_taxonomy' => true,
			] ),
		] );

		$fm->add_meta_box( __( 'Primary Category' ), [ 'post' ], 'side' );
	}
}

/**
 * Get the singleton instance.
 *
 * @return Primary_Category_Post_URL
 */
function primary_category_post_url() {
	return Primary_Category_Post_URL::instance();
}
add_action( 'after_setup_theme', 'primary_category_post_url' );