[OT] Cant write across filesystem mounts?

Sat May 30 21:20:05 UTC 2015

Hi I dont think this is specific to nginx but I hope its a good
place to ask!

When running PHP script through Nginx it writes OK to files
on the same disk mount where the PHP file is located but
not to the other parts of the system that are on another mount.
(well i dont know if its a matter of "same mount" or not, but
that is how it is behaving)

Example, /tmp is on another mount than the web root.

<?php
ini_set('display_errors', 'On');
file_put_contents('/tmp/test', 'hello world');
system('touch /tmp/test-touch');
file_put_contents('/webroot/tmp/test', 'hello world');
system('touch /webroot/tmp/test-touch');
?><html><body>hello world</body></html>

I run this script from CLI (sudo as ANY user including the php
user) and it always works fine (writes files in both places). If I
access it from a browser the write/touch commands to /tmp
fail silently. 

No AVC from selinux, no PHP or Nginx errors or warnings.
/tmp permissions are usual 777. Can someone help me in
right direction?