<?php
function HTTP_Headers_Last_Modified() {
  
  if ( ( defined( 'DOING_AJAX' ) && DOING_AJAX ) || ( defined( 'XMLRPC_REQUEST' ) && XMLRPC_REQUEST ) || ( defined( 'REST_REQUEST' ) && REST_REQUEST ) || ( is_admin() ) ) {
    return;
  }
  
  $last_modified = '';
  
  if ( is_singular() ) {
    global $post;
    
    if ( !isset( $post -> post_modified_gmt ) ) {
      return;
    }
    
    $post_time = strtotime( $post -> post_modified_gmt );
    $modified_time = $post_time;
    
    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 ( 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_sent() ) {
    return;
  }
  
  if ( !empty( $last_modified ) ) {
    
    header( 'Last-Modified: ' . $last_modified );
    
    if ( isset( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) && strtotime( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) >= $modified_time ) {
      $protocol = (isset( $_SERVER['SERVER_PROTOCOL'] ) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.1');
      header( $protocol . ' 304 Not Modified' );
    }
  }
}