Is this code displaying randomly all over your forum or just under specific posts by users? It's possible a user is trying to insert code via their signature or within a post if it only displays while showing a post.
If it shows all over the forums it'd seem that some files on your server have been modified, as this is just not something we use in the development package:
You should check the last edit time of all of your /script and /templates/default files and see if anything stands out.
Our backend processing engine (SMARTY) is likely seeing this evaled code and is blocking it from running on your forums and simply displaying it versus running it.
PHP Function evaleval — Evaluate a string as PHP code
Caution The eval() language construct is very dangerous because it allows execution of arbitrary PHP code. Its use thus is discouraged. If you have carefully verified that there is no other option than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.
Basically it seems like this code is trying to allow someone to submit arbitrary code to your forums via the $post parameter
We'd really need to see this occurring in the wild in order to be able to fully see what's going on.
