function includemyfile($filename='default')
{
  $root = "c:\\php\\";
  $prefix = "myspecialfile_";

  $target = "$root$prefix$filename";

  if ( ( ctype_alnum($filename)) && (file_exists("$target")))  {
    print "Including $filename ...\n";
  } else {
    print "BAD FILENAME!\n";
  }
}