Force encode from US-ASCII to UTF-8 (iconv)

I'm trying to transcode a bunch of files from US-ASCII to UTF-8.

For that, I'm using iconv:

iconv -f US-ASCII -t UTF-8 file.php > file-utf8.php

My original files are US-ASCII encoded, which makes the conversion not happen. Apparently it occurs because ASCII is a subset of UTF-8...

iconv US ASCII to UTF-8 or ISO-8859-15

And quoting:

There's no need for the textfile to appear otherwise until non-ASCII characters are introduced

True. If I introduce a non-ASCII character in the file and save it, let's say with Eclipse, the file encoding (charset) is switched to UTF-8.

In my case, I'd like to force iconv to transcode the files to UTF-8 anyway. Whether there is non-ASCII characters in it or not.

Note: The reason is my PHP code (non-ASCII files...) is dealing with some non-ASCII string, which causes the strings not to be well interpreted (french):

Il était une fois... l'homme série animée mythique d'Albert

Barillé (Procidis), 1ère

...

• US ASCII -- is -- a subset of UTF-8 (see Ned's answer below)
• Meaning that US ASCII files are actually encoded in UTF-8
• My problem came from somewhere else