Local/Live Problem

The exec() function falls into the same category as the shell_exec() (and passthru(),system() are all very similar) functions where you asked about a few days ago. Those functions should to be disabled for security reasons in a shared hosting environment.

Long story short, if you need/want such a function you would have to reach into your pockets and get a VPS which so you have full controll over all the server settigns and enable such a function in the servers php.ini file.