"Virtual" shell, ie. jailing an user inside a process

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http://unix.stackexchange.com – The title might not say much about the issue at hand so let me get straight to the point. Let's assume I have a casual user who can log in to the system via SSH into a bash shell. I also have a PHP (though the language is irrelevant here) script that acts as a process accepting various commands and other user input and acts according to them (essentially a 'shell-like' script). Now, what I want to do is to lock the user inside said PHP script, ie. run it as soon as the user logs in (this part is simple via .bashrc) but at the same time ensure that when the script execution ends, the user is (HowTos)