- var_dump: documentation ( source)
- preg_replace: documentation ( source)
<?php
$xmlFile = "<Font
html:Face=\"Arial\" x:Family=\"Swiss\" html:Color=\"#000000\">";
$xmlFile = preg_replace('/<Font\s+.+>/', '<Font $1 xmlns=\"http://www.w3.org/TR/REC-html40\">', $xmlFile);
var_dump($xmlFile);