1 bootstrap.inc backdrop_serve_page_from_cache(stdClass $cache)

Sets HTTP headers in preparation for a cached page response.

The headers allow as much as possible in proxies and browsers without any particular knowledge about the pages. Modules can override these headers using backdrop_add_http_header().

If the request is conditional (using If-Modified-Since and If-None-Match), and the conditions match those currently in the cache, a 304 Not Modified response is sent.


core/includes/bootstrap.inc, line 1726
Functions that need to be loaded on every Backdrop request.


function backdrop_serve_page_from_cache(stdClass $cache) {
  $page_content = $cache->data['body'];

  // Negotiate whether to use compression.
  $page_compression = !empty($cache->data['page_compressed']);
  $return_compressed = $page_compression && isset($_SERVER['HTTP_ACCEPT_ENCODING']) && strpos($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') !== FALSE;

  // Get headers set in hook_boot(). Keys are lower-case.
  $hook_boot_headers = backdrop_get_http_header();

  // Headers generated in this function, that may be replaced or unset using
  // backdrop_add_http_headers(). Keys are mixed-case.
  $default_headers = array();

  foreach ($cache->data['headers'] as $name => $value) {
    // In the case of a 304 response, certain headers must be sent, and the
    // remaining may not (see RFC 2616, section 10.3.5). Do not override
    // headers set in hook_boot().
    $name_lower = strtolower($name);
    if (in_array($name_lower, array('content-location', 'expires', 'cache-control', 'vary')) && !isset($hook_boot_headers[$name_lower])) {
      backdrop_add_http_header($name, $value);

  // If the client sent a session cookie, a cached copy will only be served
  // to that one particular client due to Vary: Cookie. Thus, do not set
  // max-age > 0, allowing the page to be cached by external proxies, when a
  // session cookie is present unless the Vary header has been replaced or
  // unset in hook_boot().
  $max_age = !isset($_COOKIE[session_name()]) || isset($hook_boot_headers['vary']) ? config_get('system.core', 'page_cache_maximum_age') : 0;
  $default_headers['Cache-Control'] = 'public, max-age=' . $max_age;

  // Entity tag should change if the output changes. Quotes are required per
  // header specification:
  // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
  $etag = '"' . $cache->created . ($return_compressed ? '-gzip' : '') . '"';
  header('Etag: ' . $etag);

  // See if the client has provided the required HTTP headers.
  $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) : FALSE;
  $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : FALSE;

  // Handle an exact match based on Etag. The If-None-Match header takes full
  // precedence over If-Modified-Since header.
  if ($if_none_match == $etag) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');

  // Handle a loose match based on date only, if no Etag was provided.
  if (!$if_none_match && $if_modified_since && $if_modified_since >= $cache->created) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');

  // Send the remaining headers.
  foreach ($cache->data['headers'] as $name => $value) {
    backdrop_add_http_header($name, $value);

  $default_headers['Last-Modified'] = gmdate(DATE_RFC1123, $cache->created);

  // HTTP/1.0 proxies does not support the Vary header, so prevent any caching
  // by sending an Expires date in the past. HTTP/1.1 clients ignores the
  // Expires header if a Cache-Control: max-age= directive is specified (see RFC
  // 2616, section 14.9.3).
  $default_headers['Expires'] = 'Fri, 16 Jan 2015 07:50:00 GMT';


  // Allow HTTP proxies to cache pages for anonymous users without a session
  // cookie. The Vary header is used to indicates the set of request-header
  // fields that fully determines whether a cache is permitted to use the
  // response to reply to a subsequent request for a given URL without
  // revalidation. If a Vary header has been set in hook_boot(), it is assumed
  // that the module knows how to cache the page.
  if (!isset($hook_boot_headers['vary']) && !settings_get('omit_vary_cookie')) {
    header('Vary: Cookie');

  if ($page_compression) {
    header('Vary: Accept-Encoding', FALSE);
    // If page_compression is enabled, the cache contains gzipped data.
    if ($return_compressed) {
      // $cache->data['body'] is already gzip'ed, so make sure
      // zlib.output_compression does not compress it once more.
      ini_set('zlib.output_compression', '0');
      header('Content-Encoding: gzip');
    else {
      // The client does not support compression, so unzip the data in the
      // cache. Strip the gzip header and run uncompress.
      $page_content = gzinflate(substr(substr($page_content, 10), 0, -8));

  // Send the final headers that indicate the page total length, after the
  // decompression of the gzipped HTML.
  $page_length = strlen($page_content);

  // Print the page. If doing an HTTP HEAD request on some servers like
  // Apache, it will abort PHP immediately once content has been returned
  // (because no additional headers would be allowed after that point). So we
  // print nothing for HEAD requests so that background processing may continue
  // if needed. Note that the content is still saved in the page cache even
  // though it is not printed to the page. See backdrop_page_footer().
  if ($_SERVER['REQUEST_METHOD'] !== 'HEAD') {
    print $page_content;