Apache service on Ubuntu 17.04 is configured by systemd with PrivateTmp=true option. You can see it in:
This has some implications - as per documentation it
"makes sharing between processes via /tmp or /var/tmp impossible".
Basically /tmp and /var/tmp will be re-mounted for the apache process after it starts.
During service restart directories will be cleaned up and re-created.
In the main filesystem they will be visible under something like:
Let's see it in action - I will find any of my apache processes running:
and query /proc for the mount information:
And surely enough - a file "cli.html" created by the apache process is then visible for me as: