GitHub always provides a link to a file with the source code. No matter what language you use. More specifically, it links to the code of a tag (a named commit).
As some languages requires compilation (e.g.: C code may require the compilation of the source code into an executable file), GitHub provides the option to upload any file as a release, in addition to the source code. Scott opted to upload a file with the intention of removing some developer-oriented files like the .gitignore or the .travis files. I see he accidentally added qa-src directory, though.
Anyway, as PHP doesn't really require compilation for its distribution as a ready-to-use package then downloading any of them would be the same. I'd prefer the link to the source as I'd be 100% sure that I'm using the real code from the tagged commit rather than a processed version of it, which adds an additional step that might result in human mistake.
