Well, I could say only miniBB traffic is full of such stuff and even more :-)
For example from our logs (minibbtest
The hackers will send such requests always, and there is nothing dangerous until you have safe up-to-date version of PHP, probably mySQL and the latest release of miniBB of course.
The code you are reffering in that case will try to execute on the malicious server and provide some info about this server to the hacker, like disk space available and system information. I suppose this
code does nothing dangerous and just checks. It works probably only if there's some hole in PHP root code which has been discovered in the past.
You could check your system for the basic security executing _install.php file which comes by default in miniBB package, with the 'analysis' parameter, i.e.
It should give information regarding PHP version, register_globals, safe_mode and vulnerable folder. These are the basic things to know to be protected.
I think having just what you see in your logs is not reason of worrying. But if you see some strange unknown files under your forums folder which do not come by default with miniBB, it is worth to investigate where they come from.