Description:
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My current production set up is PHP 5.3.10 and Apache 2.2.22, both of which I downloaded source for and compiled myself on OSX 10.6.

I've now downloaded and compiled the source for PHP 5.3.4 
and using this with Apache 2.4.2 

I have zlib output compression enabled in php.ini.

I'm using the same php.ini with both the 5.3.10 and 5.4.3 versions.

With the 5.3.4 version, the zlib compression doesn't seem to be working.  There is no http header "Content-Encoding: gzip" in my generated pages, which there is when I use the  5.3.10 software.  Pages also taking much longer to deliver and load, which again suggests that compression is not working.

From the phpinfo output, the zlib settings are the same, but I notice that php5.3.10 has zlib version 1.2.3.3, yet php 5.4.3 has zlib 1.2.3
5.3.10 :
=======
zlib
ZLib Support 	enabled
Stream Wrapper support 	compress.zlib://
Stream Filter support 	zlib.inflate, zlib.deflate
Compiled Version 	1.2.3.3
Linked Version 	1.2.3.3

Directive	Local Value	Master Value
zlib.output_compression	On	On
zlib.output_compression_level	-1	-1
zlib.output_handler	no value	no value

PHP 5.4.3
=========
zlib
ZLib Support	enabled
Stream Wrapper 	compress.zlib://
Stream Filter 	zlib.inflate, zlib.deflate
Compiled Version 	1.2.3
Linked Version 	1.2.3

Directive	Local Value	Master Value
zlib.output_compression	On	On
zlib.output_compression_level	-1	-1
zlib.output_handler	no value	no value

Also from the phpinfo pages, with 5.3.10 I see differencein the HTTP HEADERS INFORMATION (HTTP response headers) section :

PHP 5.3.10 :
===========
X-Powered-By 	PHP/5.3.10
Content-Encoding 	gzip
Vary 	Accept-Encoding

PHP 5.4.3 :
============
X-Powered-By 	PHP/5.4.3