移动括号:

if (($fd = fopen($file, "a")) !== false) { 
  fwrite($fd, 'message to be written' . "\n");   
  fclose($fd); 
}

您的代码将(fopen($file, "a") !== false)(即布尔值)的结果分配给$fd.

Your code assigned the result of (fopen($file, "a") !== false) (i.e. a boolean value) to $fd.