kellenmace/km-remove-slug-from-custom-post-type.php

## km-remove-slug-from-custom-post-type.php
<?php
/**
 * Plugin Name: Remove Slug from Custom Post Type
 * Description: Remove slug from custom post type URLs.
 * Version:     0.1.0
 * Author:      Kellen Mace
 * Author URI:  https://kellenmace.com/
 * License:     GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

/**
 * Remove the slug from published post permalinks. Only affect our custom post type, though.
 *
 * @param  string  $post_link The post permalink (URL).
 * @param  WP_Post $post      The post object.
 *
 * @return string $post_link The post permalink (URL), possibly modified.
 */
function km_remove_cpt_slug( $post_link, $post ) {
	if ( 'my-cpt-slug' === $post->post_type && 'publish' === $post->post_status ) {
		$post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );
	}

	return $post_link;
}
add_filter( 'post_type_link', 'km_remove_cpt_slug', 10, 2 );

/**
 * Have WordPress match postname to any of our public post types (post, page, my-cpt-slug).
 * All of our public post types can have /post-name/ as the slug, so they need to be unique across all posts.
 * By default, WordPress only accounts for posts and pages where the slug is /post-name/.
 *
 * @param $query The current query.
 */
function km_add_cpt_post_names_to_main_query( $query ) {
	// Bail if this is not the main query, the query does not match our rewrite rule,
	// or if we're not querying based on the post name.
	if (
		! $query->is_main_query()
		|| ! isset( $query->query['page'] )
		|| 2 !== count( $query->query )
		|| empty( $query->query['name'] )
	) {
		return;
	}

	// Add CPT to the list of post types WP will include when it queries based on the post name.
	$query->set( 'post_type', array( 'post', 'page', 'my-cpt-slug' ) );
}
add_action( 'pre_get_posts', 'km_add_cpt_post_names_to_main_query' );