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.