PHP exec() vs system() vs passthru()

What are the differences? Is there a specific situation or reason for each function? If yes, can you give some examples of those situations? PHP.net says that they are used to execute external programs. [see reference][1] From the examples I see, I don't see any obvious difference. If I were to simply run a script (bash or python), which function do you recommend me to use? [1]: http://php.net/manual/en/function.exec.php