I know what computation is in some vague sense (it is the thing computers do), but I would like a more rigorous definition. 

Dictionary.com's definitions of computation, computing, calculate, and compute are circular, so it doesn't help. 

Wikipedia defines computation to be "any type of calculation that follows a well-defined model." 
It defines calculation as "the deliberate process that transforms one or more inputs into one or more results, with variable change." But it seems this definition includes many actions as computations even though they aren't typically thought of as computation. 

For example, wouldn't this entail that, say, a bomb exploding is a computation, with the input being the fuse being lighted and the output being the explosion?

So, what exactly is computation?


Question: PHP EDI X12 Parsing
Answer: I was looking for something like that for .NET ... and nothing (at least not free, but really expensive). I don't think that you will find anything for php. I've ended up with simple code to do parsing (edi 850) and extract info that I needed. I think that it would be the best approach for you as well.