PHP XML parse - can it be faster?

You can speed up the XML parsing a little by expanding the node from XMLReader to a DOM element node, the element node can be converted into an SimpleXMLElement.

$xml_reader = new XMLReader;
$xml_reader->open("feed.xml");

$dom = new DOMDocument();
$xpath = new DOMXPath($dom);

// move the pointer to the first product
while ($xml_reader->read() && $xml_reader->name != 'SHOPITEM');

// loop through the products
while ($xml_reader->name == 'SHOPITEM') {
  // expand to DOM element
  $element = $xml_reader->expand($dom);

  // use Xpath with the $element as context node
  $itemId = $xpath->evaluate('string(ITEM_ID)', $element);

  // or import it into a SimpleXMLElement
  $item = simplexml_import_dom($element);

  //SELECT, UPDATE/INSERT HERE

  // move the pointer to the next product
  $xml_reader->next('SHOPITEM');
}
// don’t forget to close the file
$xml_reader->close();

To speed up the process you should collect the data and do mass inserts. Most DBMS support it this on way or another. Less SQL queries means less work on the database, but you need memory to collect the data. You will have to find a balance.

Another possibility is to generate a local file containing all the SQL statements and executing it using a console client. This can be really fast, but it is a potential security risk.

I cannot say whether or not the DOMDocument methods would be quicker than SimpleXML or not, but to use DOMDocument you would do something like below (untested):

$xmlfile='/path/to/feed.xml';

if( file_exists( $xmlfile ) ){
    libxml_use_internal_errors( TRUE );
    $dom = new DOMDocument('1.0','utf-8');
    $dom->validateOnParse=false;
    $dom->standalone=true;
    $dom->preserveWhiteSpace=true;
    $dom->strictErrorChecking=false;
    $dom->substituteEntities=false;
    $dom->recover=true;
    $dom->formatOutput=false;
    $dom->loadXML( $xmlfile );
    $parse_errs=serialize( libxml_get_last_error() );
    libxml_clear_errors();

    $xpath=new DOMXPath( $dom );

    $items = $dom->getELementsByTagName('SHOPITEM');

    foreach( $items as $node ){

        echo $node->nodeValue;

        /* Each node is likely to have children */
        $children=$xpath->query('PRODUCTNO',$node);
        foreach( $children as $child ){
            echo $child->nodeValue;
        }

        /*
            mysql cmds
            ----------
            select , update, insert
        */
    }

    $dom=null;
}