Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
In one word, how would you sum up the difference between HTTP, HTTPS, GOPHER and FTP versus NEWS, NNTP, MAILTO and IRC? Functionally speaking, it makes sense to request an image file from the former, but not the latter.
It serves documents, I can get images from it, my web browser supports it natively (although IE does not). Ex. gopher://gopher.quux.org/g/Software/Gopher/screenshots/lynx.gif