Apache service on Ubuntu 17.04 is configured by systemd with PrivateTmp=true option. You can see it in:

$ cat /etc/systemd/system/multi-user.target.wants/apache2.service
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target

[Service]
Type=forking
Environment=APACHE_STARTED_BY_SYSTEMD=true
ExecStart=/usr/sbin/apachectl start
ExecStop=/usr/sbin/apachectl stop
ExecReload=/usr/sbin/apachectl graceful
PrivateTmp=true
Restart=on-abort

[Install]
WantedBy=multi-user.target

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: /tmp/systemd-private-3aa821d7448d4b21a2c379847aa20e20-apache2.service-0XM9bk

Let's see it in action - I will find any of my apache processes running:

$ ps auxf | grep apache
root     11593  0.0  0.1 429640 36440 ?        Ss   15:15   0:00 /usr/sbin/apache2 -k start
www-data 11598  0.0  0.2 439860 83336 ?        S    15:15   0:05  \_ /usr/sbin/apache2 -k start

and query /proc for the mount information:

$ cat /proc/11598/mountinfo | grep /tmp
316 214 8:2 /tmp/systemd-private-3aa821d7448d4b21a2c379847aa20e20-apache2.service-0XM9bk/tmp /tmp rw,relatime shared:291 master:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered
317 214 8:2 /var/tmp/systemd-private-3aa821d7448d4b21a2c379847aa20e20-apache2.service-oKEULP/tmp /var/tmp rw,relatime shared:292 master:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered

$ cat /proc/11598/mounts | grep /tmp
/dev/sda2 /tmp ext4 rw,relatime,errors=remount-ro,data=ordered 0 0
/dev/sda2 /var/tmp ext4 rw,relatime,errors=remount-ro,data=ordered 0 0

And surely enough - a file "cli.html" created by the apache process is then visible for me as:

$ ls /tmp/systemd-private-3aa821d7448d4b21a2c379847aa20e20-apache2.service-0XM9bk/tmp
cli.html