Note to this report.

The reason trans sid does not work is related to output handler registration order.  This is not a simple fix at all.  (I might set status to Suspended)

User should make sure *not* to use session auto start to make trans sid when zlib.ouput compression is enabled. 

The reason SID is not defined is not related to output control.  
I also made a patch to define SID constant always, but it's not applied to CVS. (yet) 

If you have problem with SID not defined *even if* you have used
--enable-trans-sid when you configure PHP and session.trans_sid = 1 in your php.ini, please submit a new bug report as session problem.

> User should make sure *not* to use session auto start to make trans sid
> when zlib.ouput compression is enabled. 

should be

User should make sure *not* to use session auto start to make trans sid work when zlib.ouput compression is enabled. 

Could you try to start session manually in script and report the result?
Does it work now? I think it should.

I've tried a number of different combinations of both php.ini settings (zlib.output_compression and session.auto_start) as well as in-line function calls (session_start, ob_start("ob_gzhandler")), and in every situation I've found, the transient SID is lost when compression is turned on.

Note that I'm using PHP 4.1.1 (4.1.2 is compiling as I type this) on RedHat Linux 7.2, and did compile it with --enable-trans-sid.

Here's a summary of the 5 different tests I tried:

[Test 1] 
php.ini:
	zlib.output_compression = Off
	session.auto_start = 0	
--snip--
<?
   session_start();
   session_register("testvar");
   $testvar = "test value";
?>

<a href="/test.php">Testing</a>
--snip--

Result:  PHPSESSID gets appended correctly. 


[Test 2]
php.ini:
	zlib.output_compression = Off
	session.auto_start = 1	
--snip--
<?
   session_register("testvar");
   $testvar = "test value";
?>

<a href="/test.php">Testing</a>
--snip--

Result:  PHPSESSID gets appended correctly.


[Test 3]
php.ini:
	zlib.output_compression = Off
	session.auto_start = 0	
--snip--
<?
   ob_start("ob_gzhandler");
   session_start();
   session_register("testvar");
   $testvar = "test value";
?>

<a href="/test.php">Testing</a>
--snip--

Result:  PHPSESSID is lost and not appended.


[Test 4]
php.ini:
	zlib.output_compression = On
	session.auto_start = 0
--snip--
<?
   session_start();
   session_register("testvar");
   $testvar = "test value";
?>

<a href="/test.php">Testing</a>
--snip--

Result:  PHPSESSID is lost and not appended.


[Test 5]
php.ini:
	zlib.output_compression = On
	session.auto_start = 1
--snip--
<?
   session_register("testvar");
   $testvar = "test value";
?>

<a href="/test.php">Testing</a>
--snip--

Result:  PHPSESSID is lost and not appended.

Works like a charm!

Just tried 4.1.2 using ob_start("ob_gzhandler") and session_start(), and the SID was passed perfectly.

I'll fiddle with the php.ini settings too and see how we do, but that's definitely good enough for me.

Kudos!

One last update...  :)

Using the php.ini values to turn session.auto_start and zlib.output_compression also appear to work great.

4.1.2 truly appears to fix this problem.

Thank for feedback.
I'll update doc later so that user know what is going on.