remove-custom-post-type-slug-from-url.php

<?php
/**
 * Remove custom post type slug from URL.
 */
function pdvn_remove_slug( $post_link, $post, $leavename ) {

  if ( 'product' != $post->post_type || 'publish' != $post->post_status ) {
    return $post_link;
  }

  $post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );
  // $post_link = str_replace( '/san-pham/', '/', $post_link ); If used: $args['rewrite'] = array( 'slug' => 'san-pham' );

  return $post_link;
}
add_filter( 'post_type_link', 'pdvn_remove_slug', 10, 3 );

/**
 * [pdvn_parse_request description]
 */
function pdvn_parse_request( $query ) {

  if ( ! $query->is_main_query() || 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
    return;
  }

  if ( ! empty( $query->query['name'] ) ) {
    $query->set( 'post_type', array( 'post', 'product', 'page' ) );
  }
}
add_action( 'pre_get_posts', 'pdvn_parse_request' );