Stacker Nobody asked about the most shocking thing new programmers find as they enter the field.
Very high on the list, is the impact of inheriting a codebase with which
create documentation for each thing you figured out from the codebase. find out how it works by exprimentation - changing a few lines here and there and see what happens. use geany as it speeds up the searching of commonly used variables and functions in the program and adds it to autocomplete. find out if you can contact the orignal developers of the code base, through facebook or through googling for them. find out the original purpose of the code and see if the code still fits that purpose or should be rewritten from scratch, in fulfillment of the intended purpose.
find out what frameworks did the code use, what editors did they use to produce the code.
the easiest way to deduce how a code works is by actually replicating how a certain part would have been done by you and rechecking the code if there is such a part.
it's reverse engineering - figuring out something by just trying to reengineer the solution.
most computer programmers have experience in coding, and there are certain patterns that you could look up if that's present in the code.
there are two types of code, object oriented and structurally oriented.
if you know how to do both, you're good to go, but if you aren't familiar with one or the other, you'd have to relearn how to program in that fashion to understand why it was coded that way.
in objected oriented code, you can easily create diagrams documenting the behaviors and methods of each object class.
if it's structurally oriented, meaning by function, create a functions list documenting what each function does and where it appears in the code..
i haven't done either of the above myself, as i'm a web developer it is relatively easy to figure out starting from index.php to the rest of the other pages how something works.
goodluck.