gistfile1.phtml

The first is to use the object oriented way:
$config = array(
           'indent'         => true,
           'output-xml'   => true,
           'wrap'           => 200);

// Tidy
$tidy = new tidy;
$tidy->parseString($xml, $config, 'utf8');
$tidy->cleanRepair();
$result = (string) $tidy;

But I get the error:
Fatal Error: Class ‘tidy’ not found

The second is to just call the function like so:
$result = tidy_parse_string($xml, $config, 'utf8');

However then I get the error:
tidy_parse_string() expects exactly 1 parameter 3 given