WP::handle_404()

	public function handle_404() {
		global $wp_query;

		/**
		 * Filters whether to short-circuit default header status handling.
		 *
		 * Returning a non-false value from the filter will short-circuit the handling
		 * and return early.
		 *
		 * @since WP-4.5.0
		 *
		 * @param bool     $preempt  Whether to short-circuit default header status handling. Default false.
		 * @param WP_Query $wp_query ClassicPress Query object.
		 */
		if ( false !== apply_filters( 'pre_handle_404', false, $wp_query ) ) {
			return;
		}

		// If we've already issued a 404, bail.
		if ( is_404() )
			return;

		// Never 404 for the admin, robots, or if we found posts.
		if ( is_admin() || is_robots() || $wp_query->posts ) {

			$success = true;
			if ( is_singular() ) {
				$p = false;

				if ( $wp_query->post instanceof WP_Post ) {
					$p = clone $wp_query->post;
				}

				// Only set X-Pingback for single posts that allow pings.
				if ( $p && pings_open( $p ) ) {
					@header( 'X-Pingback: ' . get_bloginfo( 'pingback_url', 'display' ) );
				}

				// check for paged content that exceeds the max number of pages
				$next = '<!--nextpage-->';
				if ( $p && false !== strpos( $p->post_content, $next ) && ! empty( $this->query_vars['page'] ) ) {
					$page = trim( $this->query_vars['page'], '/' );
					$success = (int) $page <= ( substr_count( $p->post_content, $next ) + 1 );
				}
			}

			if ( $success ) {
				status_header( 200 );
				return;
			}
		}

		// We will 404 for paged queries, as no posts were found.
		if ( ! is_paged() ) {

			// Don't 404 for authors without posts as long as they matched an author on this site.
			$author = get_query_var( 'author' );
			if ( is_author() && is_numeric( $author ) && $author > 0 && is_user_member_of_blog( $author ) ) {
				status_header( 200 );
				return;
			}

			// Don't 404 for these queries if they matched an object.
			if ( ( is_tag() || is_category() || is_tax() || is_post_type_archive() ) && get_queried_object() ) {
				status_header( 200 );
				return;
			}

			// Don't 404 for these queries either.
			if ( is_home() || is_search() || is_feed() ) {
				status_header( 200 );
				return;
			}
		}

		// Guess it's time to 404.
		$wp_query->set_404();
		status_header( 404 );
		nocache_headers();
	}