How to disable PHP functions globally via Plesk

Question

How to disable PHP functions globally via Plesk?
E.g. in order to restrict PHP mail() functionality.

Answer

Note: Instructions below are for the default Plesk installation with vanilla PHP. It will not work for the Plesk with CageFS or Alt-PHP.

Disabling PHP functions is possible on a handler level. To achieve this, perform the following:

1. Log into Plesk and go to Tools & Settings > General Settings > PHP Settings;

2. Choose PHP handler which settings you wish to modify:
   

3. Switch to tab php.ini and find the directive disable_functions:
   

4. Append to the line functions so it looks like this:

   disable_functions = exec, system, mail

5. Save the changes by clicking OK.