show_source() would show source code of a PHP file using syntax highlighting for the various bits of the program. You won't have any PHP, so there would be no syntax highlighting. (There's no mention of show_source() taking URLs, and as the previous sentence notes, there wouldn't be much point if it did.)
So you might as well just use the file functions to read the page into a string, put it through htmlspecialchars() to stop it being rendered as HTML, and then echo it inside <pre> tags.
But depending on the browser being used, a person can view the source for themselves by going to the page in question and using the browser's "show source". Depending on the browser, it can even have syntax-highlighted HTML.
Incidentally, "wrong parameter count" means that the function wasn't given enough pieces of information in the following () to run. Or too many.