I don't have experience in sandboxing environments, but here are my 2 cents:
I worked on Google App Engine, both Java and Python, and GAE have their own sandboxed environment. Speaking of allowing users to run code, pranks/evil users might either try to abuse your server programmatically or can do illegal transactions(via http calls) from your server. I also do not have much knowledge in PHP, so pardon me for any bad assumptions.
Abusing your server:
CPU: They could write a never ending for-loop that just consumes your CPU. They could write code with high CPU usage.
Memory: They could write code creating objects in memory and not freeing them.
What Google does:
- They have a 30 second limit to process the request, after 30 seconds, they kill the thread.
- They mention in TOS that they can take down the app if it is consuming higher CPU. Perhaps they have automated triggers that show apps not using CPU efficiently.
What you could do:
- Limit the code execution time, say 10 or 15 seconds(or even lesser) depending on your use cases.
- As the program runs, record the CPU stats of each program execution(with user name) and flag those users who are running inefficient code frequently and issue a warning/ban them.
- I'm guessing you'd start each user submitted code in a new process, limit the memory of that program, say 32 MB. Choose the number depending on the need.
Illegal network activity:
They could send email/hit servers to scrape data. It is illegal to scrape content of several sites.
Hope this helps.
PS: For some reason, I believe this question belongs on Stackoverflow. Have your tried there?