Try something like this:
if (preg_match('/youtube.com/i', strtolower($str))) {
return $str;
}
Add this after function makeLinks($str) {
Usage:
var_dump(makeLinks('http://www.youtube.com'));
var_dump(makeLinks('http://www.test.com'));
Outputs:
string 'http://www.youtube.com' (length=22)
string '<a href="http://www.test.com">http://www.test.com</a> ' (length=54)