如何在CURL下载文件时使用CURLOPT_WRITEFUNCTION

My Class for download file direct from a link:

MyClass{

          function download($link){
                ......
                $ch = curl_init($link);
                curl_setopt($ch, CURLOPT_FILE, $File->handle);
                curl_setopt($ch,CURLOPT_WRITEFUNCTION , array($this,'__writeFunction'));
                curl_exec($ch);
                curl_close($ch);
                $File->close();
                ......

            }

          function __writeFunction($curl, $data) {
                return strlen($data);            
          } 
}

I want know how to use CRULOPT_WRITEFUNCTION when download file. Above code if i remove line:

curl_setopt($ch,CURLOPT_WRITEFUNCTION , array($this,'__writeFunction'));

Then it will run good, i can download that file.But if i use CURL_WRITEFUNCTION option i can't download file.