public function callFunction($url, $postData, $method, header='')
{
  $maxRetryTimes = 3;
  $curl = curl_init();
  /******初始化请求参数start******/
  if(strtoupper($method) !== 'GET' && $postData){
    curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($postData));
  }elseif (strtoupper($method) === 'GET' && $postData){
    $url .= '?'. http_build_query($postData);
  }
  /******初始化请求参数end******/
  curl_setopt_array($curl, array(
    CURLOPT_URL => $url,
    CURLOPT_TIMEOUT => 10,
    CURLOPT_NOBODY => 0,
    CURLOPT_RETURNTRANSFER => 1
  ));
  if(method == 'POST'){
    curl_setopt($curl, CURLOPT_POST, true);
  }
  if(false == empty()){
    curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
  }
  $response = false;
  while(($response === false) && (--$maxRetryTimes > 0)){
    $response = trim(curl_exec($curl));
  }
  return $response;
}

$ch = curl_init();
$options = array(CURLOPT_URL    => 'http://www.php.net/',
CURLOPT_HEADER    => false,
CURLOPT_HEADERFUNCTION  => 'on_curl_header',
CURLOPT_WRITEFUNCTION  => 'on_curl_write'
);

function on_curl_write($ch, $data)
{
  $pid = getmypid();
  $downloadSizeRecorder = DownloadSizeRecorder::getInstance($pid);
  $bytes = strlen($data);
  $downloadSizeRecorder->downloadData .= $data;
  $downloadSizeRecorder->downloadedFileSize += $bytes;
//  error_log(' on_curl_write '.$downloadSizeRecorder->downloadedFileSize." > {$downloadSizeRecorder->maxSize} \n", 3, '/tmp/hyb.log');
  //确保已经下载的内容略大于最大限制
  if (($downloadSizeRecorder->downloadedFileSize - $bytes) > $downloadSizeRecorder->maxSize) {
    return false;
  }
  return $bytes; //这个不正确的返回，将会报错，中断下载 "errno":23,"errmsg":"Failed writing body (0 != 16384)"
}

class DownloadSizeRecorder
{
  const ERROR_FAILED_WRITING = 23; //Failed writing body
  public $downloadedFileSize;
  public $maxSize;
  public $pid;
  public $hasOverMaxSize;
  public $fileFullName;
  public $downloadData;
  private static $selfInstanceList = array();
  public static function getInstance($pid)
  {
    if(!isset(self::$selfInstanceList[$pid])){
      self::$selfInstanceList[$pid] = new self($pid);
    }
    return self::$selfInstanceList[$pid];
  }
  private function __construct($pid)
  {
    $this->pid = $pid;
    $this->downloadedFileSize = 0;
    $this->fileFullName = '';
    $this->hasOverMaxSize = false;
    $this->downloadData = '';
  }
  /**
   * 保存文件
   */
  public function saveMaxSizeData2File(){
    if(empty($resp_data)){
      $resp_data = $this->downloadData;
    }
    $fileFullName = '/tmp/http_'.$this->pid.'_'.time()."_{$this->maxSize}.download";
    if($resp_data && strlen($resp_data)>0)
    {
      list($headerOnly, $bodyOnly) = explode("\r\n\r\n", $resp_data, 2);
      $saveDataLenth = ($this->downloadedFileSize < $this->maxSize) ? $this->downloadedFileSize : $this->maxSize;
      $needSaveData = substr($bodyOnly, 0, $saveDataLenth);
      if(empty($needSaveData)){
        return;
      }
      file_put_contents($fileFullName, $needSaveData);
      if(file_exists($fileFullName)){
        $this->fileFullName = $fileFullName;
      }
    }
  }
  /**
   * 返回文件的md5
   * @return string
   */
  public function returnFileMd5(){
    $md5 = '';
    if(file_exists($this->fileFullName)){
      $md5 = md5_file($this->fileFullName);
    }
    return $md5;
  }
  /**
   * 返回已下载的size
   * @return int
   */
  public function returnSize(){
    return ($this->downloadedFileSize < $this->maxSize) ? $this->downloadedFileSize : $this->maxSize;
  }
  /**
   * 删除下载的文件
   */
  public function deleteFile(){
    if(file_exists($this->fileFullName)){
      unlink($this->fileFullName);
    }
  }
}

……
curl_setopt($ch, CURLOPT_WRITEFUNCTION, 'on_curl_write');//设置回调函数
……
$pid = getmypid();
$downloadSizeRecorder = DownloadSizeRecorder::getInstance($pid);
$downloadSizeRecorder->maxSize = $size_limit;
……
//发起curl请求
$response = curl_exec($ch);
……
//保存文件，返回md5
$downloadSizeRecorder->saveMaxSizeData2File(); //保存
$downloadFileMd5 = $downloadSizeRecorder->returnFileMd5();
$downloadedfile_size = $downloadSizeRecorder->returnSize();
$downloadSizeRecorder->deleteFile();

if($response === true){
  $response = $downloadSizeRecorder->downloadData;
}