Well, the thing with self documenting code is that within that function, you would find :

$pieces = explode('.', $image_name); $extension = array_pop($pieces); 

which is self explanatory when you have the function name since it's only two lines. When things get more complicated, you either have to wrap every few lines of code in a function with a descriptive name, or use comments where necessary.

I never understood why it should be an or/or matter, instead of and/and. Yes, make your code as much self documenting as possible, and yes, add some comments to the parts that would otherwise be rather unclear.