Advanced Search Query

function filter_search_by_multiple_categories( $query ) {

    // Only modify the main query on the front-end search results page

    if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {

        // Check if category_name is set and is an array

        if ( isset( $_GET['category_name'] ) && is_array( $_GET['category_name'] ) ) {

            // Get the array of category slugs

            $categories = $_GET['category_name'];

            // Implode the array into a comma-separated string

            // WordPress accepts comma-separated category slugs in the category_name parameter

            $category_string = implode( ',', $categories );

            // Set the category_name query variable

            $query->set( 'category_name', $category_string );

        }

    }

    return $query;

}

add_action( 'pre_get_posts', 'filter_search_by_multiple_categories' );