"Mind educating me in under what possibilities php source would be visible or downloadable from someone surfing to my webpage?"
It could happen when the ISP misconfigures the webserver so it cannot parse PHP code. In that case it will fall back to it's default way to handle files, and that is to serve them as txt files.
But more importantly, it is a shared host.
That means that any PHP script can include() or require() any other file in the documentroot (because the webserver has read access in it's entire documentroot). That means that user X can open PHP scripts from use Y and read them as he/she wishes. Ergo: zero security.