gistfile1.php

//set error handling up
libxml_use_internal_errors(true);
//path to the validation file
$validationSchema = dirname(__FILE__).DS.'docs'.DS.'tff'.DS.'XMLImport.xsd';

//set up a new document to work with
$doc = new DOMDocument();
$doc->formatOutput = true;
$doc->preserveWhiteSpace = false;

//load the xml into doc
$doc->loadXML($xml);

//check the validation schema
if($doc->schemaValidateSource(DOMDocument::load($validationSchema)->saveXml()){
    //validates so send...
}else{
    //throw an exception with an error message built from validition errors if possible
}