需要把方法封装到类里，已测试，没问题。 

    public function buildData($param){

        $data = '';

        $eol = "\r\n";

        $upload = $param['file'];

        unset($param['file']);

        foreach ($param as $name => $content) {

            $data .= "--" . $this->delimiter . "\r\n"

                . 'Content-Disposition: form-data; name="' . $name . "\"\r\n\r\n"

                . $content . "\r\n";

        }

        // 拼接文件流

        $data .= "--" . $this->delimiter . $eol

            . 'Content-Disposition: form-data; name="b.cover"; filename="' . $param['filename'] . '"' . "\r\n"

            . 'Content-Type:application/octet-stream'."\r\n\r\n";

        $data .= $upload . "\r\n";

        $data .= "--" . $this->delimiter . "--\r\n";

        return $data;

    }

    public function post($url, $fields) {

    if(isset($fields['file'])){

    $postData = $this->buildData($fields);

    $header = array(

        "Content-Type: multipart/form-data; boundary=" . $this->delimiter,

            "Content-Length: " . strlen($postData)

        );

    } else {

    $postData = $fields;

    $header = array(

    "Content-Type: multipart/form-data"

        );

    }

        $cl = curl_init();

        if(stripos($url, 'https://') !== FALSE) {

            curl_setopt($cl, CURLOPT_SSL_VERIFYPEER, FALSE);

            curl_setopt($cl, CURLOPT_SSL_VERIFYHOST, FALSE);

            curl_setopt($cl, CURLOPT_SSLVERSION, 1);

        }

        curl_setopt($cl, CURLINFO_HEADER_OUT, TRUE);

curl_setopt($cl, CURLOPT_HTTPHEADER, $header);

        curl_setopt($cl, CURLOPT_URL, $url);

        curl_setopt($cl, CURLOPT_RETURNTRANSFER, TRUE);

        curl_setopt($cl, CURLOPT_POST, TRUE);

        //curl_setopt($ch, CURLOPT_SAFE_UPLOAD, TRUE);

curl_setopt($cl, CURLOPT_POSTFIELDS, $postData);

        $content = curl_exec($cl);

        $status = curl_getinfo($cl);

        curl_close($cl);

        if (isset($status['http_code']) && $status['http_code'] == 200) {

            $content = json_decode($content, TRUE);

            return $content;

        } else {

            return array("code"=>"9","message"=>"网络错误");

        }

    }