Apache + php + php.zlib.compression problem

I'm running apache 1.3.31 with php 4.3.7. The previous system was apache
1.3.28 + php 4.3.3, and had the same problem.

Apache is serving both http and SSL pages.
mod_zip is activated for http hosts and disabled for SSL hosts.
To correct the inability of mod_gzip and mod_ssl to inter-operate I've enable
php compression on the SSL site.

Note: for the test I'm using a standard http test host with mod_gzip disabled

My problem is this :

php_value zlib.output_compression On

produces compressed php pages, but without the "Content-Encoding: gzip"
header. The consequence is that browsers are not decompressing the pages.

php_value output_handler ob_gzhandler

Works correctly.

I've access to the server of a friend. He is running apache 1.3.29 php 4.3.4
on a debian server (all binaries from debian) and doesn't have this problem.
I've compared our config files and nothing special appears.

What could be the source of the problem ?
A configuration error in apache and/or php ?
Since this system is compiled by me, am I missing a php module ?


--
Marc