I noticed that my ATOM feeds weren't valid. I had a hard time finding out what exactly it was cause I couldn't seem to find the fault.

It was an invisible character, particularly \x03 .

The fix I used was to just remove that character from the feed with the following....
// Remove ASCII character  aka \x03 aka ETX (End of TeXt)
$content = str_replace("\x03",'', $content);

But then I noticed a little further down from where I placed this was....
// Other control characters
// $content = preg_replace('#(?:[\x00-\x1F\x7F]|(?:\xC2[\x80-\x9F]))#', '', $content);

So I uncommented it as follows.....
// Other control characters
$content = preg_replace('#(?:[\x00-\x1F\x7F]|(?:\xC2[\x80-\x9F]))#', '', $content);

And that worked.

I wasn't sure if this is intentionally commented or whether this is considered a bug or not so I decided to post it.

Typo