I can confirm it now works properly for me (5.6) at HEAD.

This comment seem to indicate 5.6 indeed broke file_get_contents("compress.zlib://php://input"). I didn't delve much into the proposed workaround yet but it might give a hint of what's going on.

@epriestley yes, pretty much the same ones :

[Sat Feb 20 09:04:56.337247 2016] [:error] [pid 5801] [client 86.219.113.93:56185] PHP Warning:  file_get_contents(): cannot represent a stream of type Input as a File Descriptor in /home/w4w/domains/REDACTED/phabricator/support/PhabricatorStartup.php on line 144
[Sat Feb 20 09:04:56.350142 2016] [:error] [pid 5801] [client 86.219.113.93:56185] PHP Warning:  file_get_contents(compress.zlib://php://input): failed to open stream: operation failed in /home/w4w/domains/REDACTED/phabricator/support/PhabricatorStartup.php on line 144

Ubuntu 14.04
Apache : 2.4.16
Php : 5.6.12-1
Php being through mod_php

@epriestley Reverting both 5e375482 and 8269fd6e make me able to clone normally again.

@epriestley I will try reverting theses commits when I get a stable ssh into my phab instance and keep you posted.

I still can't git clone, but if I create an empty folder, init a repo in it, put my phab-hosted repo as origin remote and pull/merge into master, it works just fine great (just as cloning would/should)

I'm in the middle of a trip right now so can't test as much as I'd like but i'm still having the issue. On my desktop computer I can pull pretty fine the repos that were previously cloned but on this freshly reinstalled laptop I can't clone repositories, hanging forever after the git-upload-pack with connection left untouched, as the other comments stated here.

Indeed. Reverting .phui-font-fa:before { to .phui-font-fa { in chrome DevTools resolves the issue

Following up my previous comment :
This made me realise updates on my chrome were disabled. After re-enabling autoupdates on chrome (latest stable which is Version 42.0.2311.135 m ), the problem still exists.

Chrome Version 42.0.2311.90 m

Exact same in chrome