Question

This code works if file is previously expisting but if file doesn’t exist this code doesnt work.

$doc = new DOMDocument();
        $doc->version = '1.0';
        $doc->encoding = 'ISO-8859-1';
        $response = $doc->createElement('response');
        $doc->appendChild($response);
        $response_type= $doc->createElement('response_type','Yes');
        $response_id = $doc->createElement('response_id',$max_id_site);
        $response->appendChild($response_type);
        $response->appendChild($response_id);
        $doc->formatOutput = true;
        echo $doc->saveXML();
        $doc->save('$filename_xml');

updated code

$doc = new DOMDocument();
        $doc->version = '1.0';
        $doc->encoding = 'ISO-8859-1';
        $response = $doc->createElement('response');
        $doc->appendChild($response);
        $response_type= $doc->createElement('response_type','Yes');
        $response_id = $doc->createElement('response_id',$max_id_site);
        $response->appendChild($response_type);
        $response->appendChild($response_id);
        $doc->formatOutput = true;
        echo $doc->saveXML();
        if (! is_file($filename_xml)) {
            touch($filename_xml) or trigger_error("Can't Create File");
            $doc->save($filename_xml);
        }   

Answer 1

You can replace

$doc->save('$filename_xml');

with

if (! is_file($filename_xml)) {
    touch($filename_xml) or trigger_error("Can't Create File");
    $doc->save($filename_xml);
}