Setting Last Modified HTTP Header on static Home Page

Last-Modified header for visitors on the front-page

It’s useful to see how the Last-Modified header is added to the feeds, in the wp::send_header() method.

If we want to be able to use is_front_page(), then the filters wp_headers or send_header are probably applied to early.

We could instead use the template_redirect hook to target the front-page, before the headers are sent and after is_front_page() is ready.

Here’s an example:

/**
 * Set the Last-Modified header for visitors on the front-page 
 * based on when a post was last modified.
 */

add_action( 'template_redirect', function() use ( &$wp )
{
    // Only visitors (not logged in)
    if( is_user_logged_in() )
        return;

    // Target front-page
    if( ! is_front_page() )
        return;

    // Don't add it if there's e.g. 404 error (similar as the error check for feeds)
    if( ! empty( $wp->query_vars['error'] ) )
        return;

    // Don't override the last-modified header if it's already set
    $headers = headers_list();
    if( ! empty( $headers['last-modified'] ) )  
        return;

    // Get last modified post
    $last_modified = mysql2date( 'D, d M Y H:i:s', get_lastpostmodified( 'GMT' ), false );

    // Add last modified header
    if( $last_modified && ! headers_sent() )
        header( "Last-Modified: " . $last_modified . ' GMT' );

}, 1 );

Here we used the core PHP functions header(), headers_list() and headers_sent() and the WordPress core function get_lastpostmodified()

The Etag header could be added here too as the md5 of the last modified date.

We can then test it from the command line with e.g.:

# curl --head https://example.tld

or just use the shorthand parameter -I to only fetch the HTTP headers.