src/HTTP/Request2/Adapter/Curl.php
protected function callbackReadBody($ch, $fd, $length)
    {
        if (!$this->eventSentHeaders) {
            $this->request->setLastEvent(
                'sentHeaders', curl_getinfo($ch, CURLINFO_HEADER_OUT)
            );
            $this->eventSentHeaders = true;
        }
        if (in_array($this->request->getMethod(), self::$bodyDisallowed)
            || 0 == $this->contentLength || $this->position >= $this->contentLength
        ) {
            return '';
        }
        if (is_string($this->requestBody)) {
            $string = substr($this->requestBody, $this->position, $length);
        } elseif (is_resource($this->requestBody)) {
            $string = fread($this->requestBody, $length);
        } else {
            $string = $this->requestBody->read($length);
        }
        $this->request->setLastEvent('sentBodyPart', strlen($string));
        $this->position += strlen($string);
        return $string;
    }
protected function callbackReadBody($ch, $fd, $length)
    {
        if (!$this->eventSentHeaders) {
            $this->request->setLastEvent(
                'sentHeaders', curl_getinfo($ch, CURLINFO_HEADER_OUT)
            );
            $this->eventSentHeaders = true;
        }
        if (in_array($this->request->getMethod(), self::$bodyDisallowed)
            || 0 == $this->contentLength || $this->position >= $this->contentLength
        ) {
            return '';
        }
        if (is_string($this->requestBody)) {
            $string = substr($this->requestBody, $this->position, $length);
        } elseif (is_resource($this->requestBody)) {
            $string = fread($this->requestBody, $length);
        } else {
            $string = $this->requestBody->read($length);
        }
        $this->request->setLastEvent('sentBodyPart', strlen($string));
        $this->position += strlen($string);
        return $string;
    }
Callback function called by cURL for reading the request body

@param resource $ch     cURL handle
@param resource $fd     file descriptor (not used)
@param integer  $length maximum length of data to return

@return   string      part of the request body, up to $length bytes