PHP + cURL getting loading local xml while latest is retrieved

I'm trying to load an xml file from another website. I can do this using cURL using the following:

    function getLatestPlayerXML($par1) {
    $url = "http://somewebsite/page.php?par1=".$par1;
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $xmlresponse = curl_exec($ch);
    $xml = simplexml_load_string($xmlresponse);
    $xml->asXML("./userxml/".$par1.".xml");
    return $xml;
}

This works all well and good, however, the external website takes a long time to respond with the file, which is why I save the xml file to ./userxml/$par1.xml which also works. I load like this:

    function getLocalPlayerXML($par1) {
    $xml = simplexml_load_file("./userxml/".$par1.".xml");
    if($xml != False) {
        // How can I make it so that when called it only temporarily uses this file until the latest is available?
        return $xml;
    } else {
        return $getLatestPlayerXML($par1);
    }
}

The problem I am having is that I want it so when I call a single load function it first tries to load the xml from file and if it exists use that file until the latest file has been received at which point, update the page. If the file does not exist, simply wait until the latest file has been retrieved and then use that. Is even possible?