Php has its own /tmp in /tmp/systemd-private-nABCDE/tmp when accessed through nginx

Because systemd is configured to give nginx a private /tmp. If you must use the system /tmp instead for some reason then you will need to modify the .service file to read "PrivateTmp=no".

If you are running multiple sites on the server then I think you'll want to leave PrivateTmp=yes so that each site remains segregated even in it's use of temp files. Could be a security issue otherwise, I'd imagine.

Ignacio Vazquez-Abrams have the correct answer, but let me add my functional solution.

I've try "multi-user.target.wants" solution, it have worked but after restart, but at some point, PrivateTmp go back to true.Like my principal use of Apache2 is PHP, I finally edited php.ini and I've uncomment line sys_temp_dir.

By default system use temp dir assigned by function sys_get_temp_dir. Function sys_get_temp_dir will return "/tmp" but the truth is that your tmp files are storing at some path like /tmp/systemd-private-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-apache2.service-YYYYYY//tmp/*. So, what work for me was:

Edit php.ini (path can change between PHP versions)

sudo nano /etc/php/7.2/cli/php.ini

Then uncomment sys_temp_dir line

; Directory where the temporary files should be placed.; Defaults to the system default (see sys_get_temp_dir)sys_temp_dir = "/tmp"