Description:
------------
We have an application which requires Vary: Cookie for its caching scheme to work correctly. It's using the header() function to set the header during the request. With PHP 5.2.9/Apache requesting a page with both Accept-Encoding: gzip and without will correctly append Accept-Encoding to the Vary:

--
HEAD / HTTP/1.1
Host: www.easports.com
        
HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 01:06:33 GMT
Vary: Cookie,Accept-Encoding
--

--
HEAD / HTTP/1.1
Host: www.easports.com
Accept-Encoding: gzip

HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 01:07:37 GMT
Vary: Cookie,Accept-Encoding
Content-Encoding: gzip
--

With PHP 5.3.2 the Vary header is overwritten when the content is served as gzip:

---
HEAD / HTTP/1.1
Host: devsonet.dev.easports.ea.com

HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 01:10:00 GMT
Vary: Cookie,Accept-Encoding
--

--
HEAD / HTTP/1.1
Host: devsonet.dev.easports.ea.com
Accept-Encoding: gzip

HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 01:10:53 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
--

The attached diff will make the application work as expected. This was based on reverting a chunk that had changed between PHP 5.2.9 and PHP 5.2.13. This diff was clearly put in there to fix another bug so it's *not* intended to be applied; it's just for demonstration purposes