This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.

 For Windows:

http://windows.php.net/snapshots/
 
Thank you for the report, and for helping us make PHP better.

I have the same problem, using "php_value output_handler ob_gzhandler" in .htaccess produces the same warning all the time. I've been trying to bypass the problem but nothing works.

In 5.3.8 I didn't have that problem. So downgraded for now.

PHP Ini:
output_buffering = Off
zlib.output_compression = Off

This issue remains on ArchLinux running: PHP 5.4.3 (built: May 31 2012 10:40:02).

Even though zlib.output_compression is set to Off in php.ini and phpinfo(); reports this setting as being off, I still get this warning message at each run of any PHP script. Or more specific:

PHP Warning:  Unknown: output handler 'zlib output compression' conflicts with 'ob_gzhandler' in Unknown on line 0

The same issue happens both when running through lighttpd as when using the CLI. If any more information is required, please let me know which.

Seems to be a problem in merging the fix to actual branch, which releases are made of.
This commit:
http://git.php.net/?p=php-src.git;a=commit;h=85a62e9086db7a8ddfcda4ab1279a2439935f8d5
really fixes the problem, but relevant line is not present in php-5.4.3 and php-5.4.4.

You are right. It seems to be fixed in php-5.4.4-1; no more errors now. Thanks.

Hi, it's not fixed in my PHP Version: 5.4.4-2~dotdeb.0

The Line "ZLIBG(output_compression) = 0;" in ext/zlib/zlib.c from the obove commit 
is not in the actual head or 5.4.4 branch anymore.

Same problem here. This bug isn't fixed in 5.4.5RC1.

same problem open for 5.4.4:
https://bugs.php.net/bug.php?id=62335

Still present in the recent 5.4.5 update. Very frustrating.

Yes, something isn't being reset after the request when you use 
ob_start('ob_gzhandler')

Please try using this snapshot:

  http://snaps.php.net/php5.4-latest.tar.gz
 
For Windows:

  http://windows.php.net/snapshots/

Hi, I've made a fix for this, could you please try with it?

The error message is gone, but the gzip compression doesn't work no longer.

The configuration that I use in php-fpm.conf is:
php_flag[output_buffering] = on
php_flag[zlib.output_compression] = off
php_value[output_handler] = ob_gzhandler

Can someone of the devs mark the bug 62335 as duplicate.

I tested with php build server, didn't reproduce your issue.
test script: 
<?php
ob_start('ob_gzhandler');
echo 'hello, world';
ob_end_flush();
?>

server response:
HTTP/1.1 200 OK

Host: inf-dev-porsche.weibo.com:8964

Connection: close

X-Powered-By: PHP/5.5.0-dev

Content-Encoding: gzip

Vary: Accept-Encoding

Content-type: text/html

oh, I mis-understand your codes,  verified.

Here is the problem
ext/zlib/zlib.c
@@ -205,7 +205,7 @@ static int php_zlib_output_handler(void **handler_context, 
php_output_context *o
 		if (SUCCESS == 
php_output_handler_hook(PHP_OUTPUT_HANDLER_HOOK_GET_FLAGS, &flags TSRMLS_CC)) {
 			/* only run this once */
 			if (!(flags & PHP_OUTPUT_HANDLER_STARTED)) {
				if (SG(headers_sent) || 
!ZLIBG(output_compression)) {  

seems we need a bigger work to resolve this

here is the confusion(assuming -d output_handler=ob_gzhandler -d 
zlib.output_compression=0) :

1. php.output_handler will change the ZLIGB(output_compression) before the zlib 
RINIT 
2. in zlib RINIT, we set the ZLIBG(output_compression) to default value(ini)

3. if we don't override the ZLIBG(output_compression), then in the 
php_zlib_output_compression_start which will be called in RINT will try to start 
zlib compression handler (although it depends on the requeset header), then, the 
conflict warning will be threw.

4. if we override it, then it the php_zlib_output_compression_start, it will 
return FALIURE, and no compression occurred(see the codes from my previous 
reply)

so, the key problem is multi-featrues depends on one global flag -> 
ZLIBG(output_compression).

re-fixed agian...

Now, it works for me.

I want to test the mentioned snapshot but none of the links are working on this page: http://windows.php.net/snapshots/
Where have they gone? I am using Win7 x64 and this bug is driving me crazy for some time. Will it be finally fixed in PHP 5.4.6?

Problems occurs for me on :

php-5.4.13-1.fc18.x86_64 + httpd-2.4.4-2.fc18.x86_64
and
php-5.4.11-1.fc18.x86_64 + httpd-2.4.3-15.fc18.x86_64

zlib.output_compression = Off in php.ini
With apache mod_deflate on or off it is the same.

Any advise to fix it please ?

Seem to be getting this bug again in 5.5.10 (cPanel)

I'm encounter this bug again in php 7. Can anyone help?

Having the same issue - using php v7.0.10.  It is now 2020 - first reported in 2011 already...?