How to process custom code?

Question

PHP

Two options for processing PHP code:
1) eval() - brutal hardcore! I don't even think it's worth considering.
2) Sending the code to the server and executing it there with getting the result.
And now a question on the second point:
1) How to process the data on the server? I may not understand correctly what "process on the server" means, in the process of answering, I will correct the question.
2) What "pitfalls" can be encountered? For example, looping.
PS: what am I going to do: Get the code from the user, execute it, get the result.

Answer 1

Tell me, Mr. Babbage, if you enter the wrong data into the machine, will you get the right answer?

I find your question very similar.

Answer 2

What for?
In general, first you need to figure out what a site is .