PHP can't read file on /var/tmp
Fedora 16 introduced the concept of a "private tmp" /tmp and /var/tmp I would throw my 2 cents on that direction.
2.3.3. Services Private /tmp
A number of services managed by systemd have been modified to make use of its ability to provide them with a private /tmp directory. Privileged services using /tmp and /var/tmp have previously been found to be open to being interfered with by unprivileged users, potentially leading to privilege escalation. Using private /tmp directories for services prevents this style of exploit.The directive added to the systemd unit files for the modified services is:
[Service]
PrivateTmp=true