Jailkit is used to create a secure shell environment for the user. It's also used by ISPConfig to securely run user's cron jobs from command line. Default jailkit configuration may not properly support PHP. So, first jailkit needs to be installed:
$ sudo apt-get install jailkitThen configuration file /etc/jailkit/jk_init.ini needs to be modified by addding 2 new sections - php and env: