Debug DOM: file XML apparentemente vuoto

[tryhere]

Ho il seguente codice PHP:

$domstatus=new DOMDocument();
$domchess=new DOMDocument();

$fpstatus=fopen("status.xml","r+");
$fpchess=fopen("chess.xml","r+");

if (flock($fpstatus,LOCK_EX)) {
    $xmlstatus=fread($fpstatus,filesize("status.xml"));
    $domstatus->loadXML($xmlstatus);
    $rootstatus=$domstatus->documentElement;
    if (($statusnodeengine=$rootstatus->getElementsByTagName("engine"))&&($statusnodeactive=$rootstatus->getElementsByTagName("active")))
        if (($statusnodeengine->item(0)->nodeValue=="on")&&($statusnodeactive->item(0)->nodeValue=="off")) {
            $statusnodeactive->item(0)->nodeValue="on";
            $xmlstatus=$domstatus->saveXML();
            fseek($fpstatus,0,SEEK_SET);
            fwrite($fpstatus,$xmlstatus);
            ftruncate($fpstatus,ftell($fpstatus));            
            if (flock($fpchess,LOCK_EX)) {
            
                $xmlchess=fread($fpchess,filesize("chess.xml")); //fread non ritorna nulla
                $domchess->loadXML($xmlchess);
                $rootchess=$domchess->documentElement;
                
                flock($fpchess,LOCK_UN);
            } //if            
            $xmlstatus=fread($fpstatus,filesize("status.xml"));
            $domstatus->loadXML($xmlstatus);
            $rootstatus=$domstatus->documentElement;
            $statusnodeactive=$rootstatus->getElementsByTagName("active");
            $statusnodeactive->item(0)->nodeValue="off";
            $xmlstatus=$domstatus->saveXML();
            fseek($fpstatus,0,SEEK_SET);
            fwrite($fpstatus,$xmlstatus);
            ftruncate($fpstatus,ftell($fpstatus));
        } //if
    flock($fpstatus,LOCK_UN);
} //if

fclose($fpchess);
fclose($fpstatus); 

Dopo l'esecuzione i file rimangono immutati (come deve essere) ma la fread() non restituisce nulla, nonostante il file non sia vuoto.
Come mai?