What is this character ( Â ) and how do I remove it with PHP?

"Latin 1" is your problem here. There are approx 65256 UTF-8 characters available to a web page which you cannot store in a Latin-1 code page.

For your immediate problem you should be able to

$clean = str_replace(chr(194)," ",$dirty)

However I would switch your database to use utf-8 ASAP as the problem will almost certainly reoccur.

This works for me:

$string = "Sentence ‘not-critical’ and \n sorting ‘not-critical’ or this \r and some ‘not-critical’ more. ' ! -.";
$output = preg_replace('/[^(\x20-\x7F)\x0A\x0D]*/','', $string);

It isn't really one character, and is likely caused by misalignment between content encoding and browser's encoding. Try to set the encoding of your outputted page to what you are using.

e.g. In the section, output:

echo "<META http-equiv='Content-Type' content='text/html; charset=UTF-8'>";

(Adjust UTF-8 to whatever you are using)