The exec function executes a system command and never returns
I really think this part of the documentation should be modified a bit. (Anyone know who to contact?)

When I first read that when I had a similar problem, I thought it meant did not return a value, and the "returns false only.." bit was put it as it is to emphasise that the function returning a value is a rare event.

I would like it be changed to:
"The exec function replaces the currently running process with the command or program called, unless the command or program does not exist. If the command or program does not exist, the call will fail and exec will return false."