diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index d597a05306d2b06dd3db89bc72fb0545152a7aff..02618277f1a3caa3453a941fbde57d5c4763a932 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -380,6 +380,16 @@ function cache_clear_all($cid = NULL, $wildcard = false) {
 
 /**
  * Store the current page in the cache.
+ *
+ * We try to store a gzipped version of the cache. This requires the
+ * PHP zlib extension (http://php.net/manual/en/ref.zlib.php).
+ * Presence of the extension is checked by testing for the function
+ * gzencode. There are two compression algorithms: gzip and deflate.
+ * The majority of all modern browsers support gzip or both of them.
+ * We thus only deal with the gzip variant and unzip the cache in case
+ * the browser does not accept gzip encoding.
+ *
+ * @see drupal_page_header
  */
 function page_set_cache() {
   global $user, $base_url;
@@ -387,16 +397,23 @@ function page_set_cache() {
   if (!$user->uid && $_SERVER['REQUEST_METHOD'] == 'GET') {
     // This will fail in some cases, see page_get_cache() for the explanation.
     if ($data = ob_get_contents()) {
+      $cache = TRUE;
       if (function_exists('gzencode')) {
-        if (version_compare(phpversion(), '4.2', '>=')) {
-          $data = gzencode($data, 9, FORCE_GZIP);
+        // We do not store the data in case the zlib mode is deflate.
+        // This should be rarely happening.
+        if (zlib_get_coding_type() == 'deflate') {
+          $cache = FALSE;
         }
-        else {
-          $data = gzencode($data, FORCE_GZIP);
+        else if (zlib_get_coding_type() == FALSE) {
+          $data = gzencode($data, 9, FORCE_GZIP);
         }
+        // The remaining case is 'gzip' which means the data is
+        // already compressed and nothing left to do but to store it.
       }
       ob_end_flush();
-      cache_set($base_url . request_uri(), $data, CACHE_TEMPORARY, drupal_get_headers());
+      if ($cache && $data) {
+        cache_set($base_url . request_uri(), $data, CACHE_TEMPORARY, drupal_get_headers());
+      }
     }
   }
 }
@@ -569,6 +586,8 @@ function drupal_set_title($title = NULL) {
 
 /**
  * Set HTTP headers in preparation for a page response.
+ *
+ * @see page_set_cache
  */
 function drupal_page_header() {
   if (variable_get('cache', 0)) {