Session cookie not being sent when PHP session.auto_start set true

I been trying to replicate a production setup on my development server but can't get the sessions to work properly.

After many frustrating hours I've tracked the problem down to the fact that php.ini directive session.auto_start isn't starting the session.

AFAIK, session.auto_start = 1 and a session_start() at the top of a script should be functionally identical.

If I call an empty PHP script using curl to show only the headers I get the following with session.auto_start = 1 (checked in php info page that it is indeed set to true).

me@myhost:~$ curl -I test.server.local
HTTP/1.1 200 OK
Date: Sat, 20 Feb 2010 05:18:05 GMT
Server: Apache
P3P: CP="NOI DSP COR NID PSA ADM OUR IND NAV COM"
Content-Type: text/html

No Session cookie.

The only change I make is to disable session.auto_start, add a session_start(); into the script and restart apache.

me@myhost:~$ curl -I test.server.local
开发者_Python百科HTTP/1.1 200 OK
Date: Sat, 20 Feb 2010 05:18:11 GMT
Server: Apache
Set-Cookie: PHPSESSID=c7bdfb8eca47c12d97ff3ad86081d470; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
P3P: CP="NOI DSP COR NID PSA ADM OUR IND NAV COM"
Content-Type: text/html

Now it works.

Can anybody help me to understand why session.auto_start isn't sending a session cookie.

This was caused by a bug in XDebug (ironic huh?). I've just disabled xdebug but I'm sure an upgrade would probably fix it as well.