diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index ed863cdf678..5cb700a9ad9 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -218,6 +218,8 @@ int php_zlib_output_handler(void **handler_context, php_output_context *output_c
 					deflateEnd(&ctx->Z);
 					return FAILURE;
 			}
+			/* "Vary: Accept-Encoding" header sent along uncompressed content breaks caching in MSIE,
+			   so let's just send it with successfully compressed content, see http://bugs.php.net/40325 */
 			sapi_add_header_ex(ZEND_STRL("Vary: Accept-Encoding"), 1, 1 TSRMLS_CC);
 			php_output_handler_hook(PHP_OUTPUT_HANDLER_HOOK_IMMUTABLE, NULL TSRMLS_CC);
 		}