$filename = 'test.xml';

// sprawdz czy istnieje i czy zapisywalny
if (is_writable($filename)) {

   // otworz plik do zapisu,ustaw wskaznik na koncu
   if (!$handle = fopen($filename, 'w')) {
         echo "Cannot open file ($filename)";
         exit;
   }

   // zapisz
   if (fwrite($handle, $newxml ) === FALSE) {
       echo "Cannot write to file ($filename)";
       exit;
   }
  
   echo "Success, wrote XML to file ($filename)<br/>";
  
   fclose($handle);

} else {
   echo "The file $filename is not writable";
}