I wrote a couple of functions to:
 - create a DOMDocument from a file
 - parse the namespaces in it
 - create a XPath object with all the namespaces registered
 - load the schemalocations
 - validate the file on the main schema (the one without prefix)

It is useful for me, see if it is also for someone else!!

Giulio

function decodeNode($node)
{
    $out = $node->ownerDocument->saveXML($node);
    $re = "{^<((?:\\w*:)?\\w*)". //the tag name
    "[\\s\n\r]*((?:[\\s\n\r]*". 
    "(?:\\w*:)?\\w+[\\s\n\r]*=[\\s\n\r]*". //possible attribute name
    "(?:\"[^\"]*\"|\'[^\']*\'))*)". //attribute value
    "[\\s\n\r]*>[\r\n]*".
    "((?:.*[\r\n]*)*)". //content
    "[\r\n]*</\\1>$}"; //closing tag
    preg_match($re, $out, $mat);
    return $mat;
}

function innerXml($node)
{
    $mat = decodeNode($node);
    return $mat[3];
}

function getnodeAttributes($node)
{
    $mat = decodeNode($node);
    $txt = $mat[2];
    $re = "{((?:\\w*:)?\\w+)[\\s\n\r]*=[\\s\n\r]*(\"[^\"]*\"|\'[^\']*\')}";
    preg_match_all($re, $txt, $mat);
    $att = array();
    for ($i=0; $i<count($mat[0]); $i++)
    {
        $value = $mat[2][$i];
        if ($value[0] == "\'" || $value[0] == "\"")
        {
            $len = strlen($value);
            $value = substr($value, 1, strlen($value)-2);
        }
        $att[ $mat[1][$i] ] = $value;
    }
    return $att;
}

function loadXml($file)
{
    $doc = new DOMDocument();
    $doc->load($file);
    //cerca l'attributo xmlns
    $xsi = false;
    $doc->namespaces = array();
    $doc->xpath = new DOMXPath($doc);
    
    $attr = getnodeAttributes($doc->documentElement);
    foreach ($attr as $name => $value)
    {
        if (substr($name,0,5) == "xmlns")
        {
            $uri = $value;
            $pre = $doc->documentElement->lookupPrefix($uri);
            if ($uri == "http://www.w3.org/2001/XMLSchema-instance")
                $xsi = $pre;
            $doc->namespaces[$pre] = $uri;
            if ($pre == "")
                $pre = "noname";
            $doc->xpath->registerNamespace($pre, $uri);
        }
    }
    
    if ($xsi)
    {
        $doc->schemaLocations = array();
        $lst = $doc->xpath->query("//@$xsi:schemaLocation");
        foreach($lst as $el)
        {
            $re = "{[\\s\n\r]*([^\\s\n\r]+)[\\s\n\r]*([^\\s\n\r]+)}";
            preg_match_all($re, $el->nodeValue, $mat);
            for ($i=0; $i<count($mat[0]); $i++)
            {
                $value = $mat[2][$i];
                $doc->schemaLocations[ $mat[1][$i] ] = $value;
            }
        }

        $olddir = getcwd();
        chdir(dirname($file));
        $schema = $doc->schemaLocations[$doc->namespaces[""]];
        if (substr($schema,0,7) == "file://")
        {
            $schema = substr($value,7);
        }
        if (!$doc->schemaValidate($schema))
            dbg()->err("Invalid file");
        chdir($olddir);
    }
    
    return $doc;
}