PHP exec () does not execute all commands

I am using PHP-FPM with Nginx on Ubuntu. I am trying to launch a PHP task from a web application.

The following two examples work correctly when I run the files from CLI, but not from Nginx.

exec("notify-send 'Starting master at ".date("h:m:s")."'");
echo exec("/home/rohit/Projects/webapp/console/index.php blast 23");

These two work correctly from both CLI and Nginx.

echo exec("whoami");
echo exec("pwd");

I tried changing the PHP-FPM user from www-data to rohit (my username), but no luck. What am I doing wrong?

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The problem is that you're running notify-send from a service. Notify-send is a desktop-oriented program which interacts with the display. But nginx runs without being attached to a display.

Imagine, for example, that there are 3 people logged on to the computer at the same time, all with different displays. When notify-send runs, it wouldn't know which display to send the notification to.