In current 1.8.4rc1 snapshot there is a problem with Content-length being wrong if PHP is configured with "zlib.output_compression = On". Content-length calculated in Zabbix is wrong because of the output compression applied later. That means, if a client accepts gzip and checks the Content-length, it will not match because less data ist sent than expected because of compression.
In my situation the following stack doesn't work:
Apache/PHP -> Pound -> Inet -> Squid -> Firefox

Attached patch fixed it for me. I've just commented out the Content-length header. A Google search told me that doing this correctly in PHP is not very easy. Does it hurt to remove the Content-length header?