Устанавливает заголовок `Last-Modified` для WordPress и умеет обрабатывать заголовок `HTTP_IF_MODIFIED_SINCE`

wordpress-last-modified.php

<?php
add_action( 'template_redirect', 'set_last_modified_headers', 999 );

function set_last_modified_headers() {

	if ( ( defined( 'DOING_AJAX' ) && DOING_AJAX )
		|| ( defined( 'XMLRPC_REQUEST' ) && XMLRPC_REQUEST )
		|| ( defined( 'REST_REQUEST' ) && REST_REQUEST )
		|| ( is_admin() ) ) {
		return;
	}

	/**
	 * if WooCommerce cart, checkout, account - just return
	 */
	if ( class_exists( 'woocommerce' ) && function_exists('is_cart') && function_exists('is_checkout') && function_exists('is_account_page')
		&& ( is_cart() || is_checkout() || is_account_page() ) ) return;

	/**
	 * if Search - just return
	 */
	if ( is_search() ) return;


	$last_modified = '';


	/**
	 * If posts, pages, custom post types
	 */
	if ( is_singular() ) {
		global $post;

		if ( ! isset($post->post_modified_gmt) ) {
			return;
		}

		$post_time = strtotime( $post->post_modified_gmt );
		$modified_time = $post_time;

		/**
		 * If we have comment set new modified date
		 */
		if ( (int) $post->comment_count > 0 ) {
			$comments = get_comments( array(
				'post_id' => $post->ID,
				'number' => '1',
				'status' => 'approve',
				'orderby' => 'comment_date_gmt',
			) );
			if ( ! empty($comments) && isset($comments[0]) ) {
				$comment_time = strtotime( $comments[0]->comment_date_gmt );
				if ( $comment_time > $post_time ) {
					$modified_time = $comment_time;
				}
			}
		}

		$last_modified = str_replace('+0000', 'GMT', gmdate('r', $modified_time));

	}

	/**
	 * If any archives: categories, tags, taxonomy terms, post type archives
	 */
	if ( is_archive() || is_home() ) {
		global $posts;

		if ( empty($posts) ) {
			return;
		}

		$post = $posts[0];

		if ( ! isset($post->post_modified_gmt) ) {
			return;
		}

		$post_time = strtotime( $post->post_modified_gmt );
		$modified_time = $post_time;

		$last_modified = str_replace('+0000', 'GMT', gmdate('r', $modified_time));
	}


	/**
	 * If headers already sent - do nothing
	 */
	if ( headers_sent() ) {
		return;
	}


	if ( ! empty($last_modified) ) {
		header( 'Last-Modified: ' . $last_modified );

		if ( ! is_user_logged_in() ) {
			if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $modified_time) {
				$protocol = (isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0');
				header($protocol . ' 304 Not Modified');
			}
		}
	}
}