Sometimes you don't want to transform to a file, an XML string (because you're using text or html), or a DOMDocument.
To transform to standard output, you can use php://output
<?php
$proc->transformToURI($xml, 'php://output');
?>
To transform to a string (of HTML or text, instead of XML), you can use the above in conjunction with output buffering.
<?php
ob_start();
$proc->transformToURI($xml, 'php://output');
$outputString = ob_get_flush();
?>