Description:
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So I created a new pool and chrooted it to /var/www/site1.

# /etc/php5/fpm/pool.d/site1.conf
# ...
chroot = /var/www/site1
chdir = /
# ...


I bind-mounted /tmp to /var/www/site1/tmp.

I changed the systemd service configuration file php5-fpm.service and added "PrivateTmp=true" under "[Service]".

When I start the php5-fpm service I can see that PrivateTmp took effect because there is a folder named /tmp/systemd-private-*-php5-fpm.service-*/

But a scandir("/tmp") lists the content of /tmp, thus PrivateTmp having no effect when chroot= is active.

But when I change the php pool config file and remove the "chroot=" configuration, a scandir("/tmp") lists the content of /tmp/systemd-private-*-php5-fpm.service-*/