Is "on" supposed to produce garbage (apparently
Content-Encoding is simply missing), while 1 is the one true
setting? I always thought they were equivalent in the
context of a boolean flag.

works

php_admin_value zlib.output_compression 1

compression, but incomplete HTTP headers

php_admin_value zlib.output_compression on

Tested with current PHP 4.3.5 tree.

From php.ini-dist:

; Transparent output compression using the zlib library
; Valid values for this option are 'off', 'on', or a specific buffer size
; to be used for compression (default is 4KB)
; Note: Resulting chunk size may vary due to nature of compression. PHP
; outputs chunks that are few hundreds bytes each as a result of
; compression. If you prefer a larger chunk size for better
; performance, enable output_buffering in addition.
; Note: You need to use zlib.output_handler instead of the standard
; output_handler, or otherwise the output will be corrupted.
zlib.output_compression = Off

So it actually shouldn't be boolean option at all..?

--Jani