Sub_Nav::add_subnav_above_content( string $content )

	public function add_subnav_above_content( $content ) {
		if ( ! is_singular() ) {
			return $content;
		}
		$post_id = get_queried_object_id();
		if ( $post_id !== get_the_ID() ) {
			return $content; // don't filter if we're not on the main post
		}
		/**
		 * Filter whether to display the gift certificate subnav before the post content
		 * on gift certificate pages.
		 *
		 * @param bool $display True to display the subnav, false to skip it
		 * @param int  $post_id The ID of the current page
		 */
		if ( ! apply_filters( 'bigcommerce/gift_certificates/do_subnav', true, $post_id ) ) {
			return $content;
		}
		switch ( $post_id ) {
			case get_option( Pages\Gift_Certificate_Page::NAME, 0 ):
			case get_option( Pages\Check_Balance_Page::NAME, 0 ):
				return $this->get_subnav() . $content;
			default:
				return $content;
		}
	}