Validating XML

Hello, Anyone have pointers to good tutorials out there for validating XML with DTD? I have looked at the top comment on: http://www.php.net/manual/en/ref.xmlreader.php#xmlreader.constants Where you set the parser property to validate, but it is kind of like a black box...what is it using the validate the XML schema? I am working with an XML document and a DTD file which is separate, do the files have to share a name with a different extension, or does the DTD somehow have to be embedded? TIA -Brad