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You mean actual PHP code that will be executed? So the user can do something like this:
<?php mkdir("/path/to/my/dir", 0700); ?>
And perhaps change it later to /path/to instead. And all this will be executed? If so, then this will not be something I will be implementing. It's something very scary to do. Users may not have access within a block to execute ANY PHP code they want.. It's unsafe. Your site can be totally destroyed by users who are not familiar with PHP or even hackers hijacking a user account and doing stuff..
If it's something else entirely you're looking for, let me know! :)