Marco.org

…with certain input that contains invalid UTF-8 characters, when added to text nodes or attribute values.

It’s probably a problem with the version of libxml included in OS X 10.5 (Leopard). It outputs successfully (including the invalid characters) with the same script on CentOS Linux.

Workaround:

$text = iconv('UTF-8', 'UTF-8//IGNORE', $text);

This will strip any invalid UTF-8 characters from $text. (I’m assuming here that $text is already supposed to be UTF-8 in your application.)