When you execute any programme through a webserver, you execute them with the permissions and as the user the webserver is running. You will be running them in the current working directory of the webserver too.

All this leads to a number of possible reasons why things aren't working the way you want them to.

The webserver might even be configured in a way not to let you use exec or other means to execute programmes locally.

Best you can do is to find out which user the webserver is running with and which current working directory it is using. Change to that user id and working directory and try on the shell level. That helps avoiding a number of problems.

The webserver's error_log file will tell you everything else, I suppose.

Cheers, Sören