从json解码中检索NULL

Question

我得到了以下代码来通过API检索一些json：

    public function get_json($jsonurl, $ttl = 3600) {
    $cache_file = $this->cache_dir . DIRECTORY_SEPARATOR . md5($jsonurl);
    $function = __FUNCTION__;

    if (file_exists($cache_file) && filemtime($cache_file) < time() - $ttl) {
        unlink($cache_file);
    }

    if (!file_exists($cache_file)) {            
        file_put_contents($this->cache_dir . DIRECTORY_SEPARATOR . 'cache.log', $jsonurl . ': ' . $cache_file . "\n", FILE_APPEND); 
        try{
            $response = $this->fetchUrl($jsonurl);
        } catch(Exception $ex){
            return true;
        }

        file_put_contents($cache_file, $response);
    }
    elseif(file_exists($cache_file)){
        $response = file_get_contents($cache_file);
    }
    $json_output = json_decode($response, true);

    if(empty($json_output)){
        unlink($cache_file);
    }

    return $json_output;
}

private function fetchUrl($url) {
    $curl = curl_init($url);
    curl_setopt_array($curl, array(
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_FOLLOWLOCATION => true
    ));
    $response = curl_exec($curl);
    if (!$response) {
        $exception = new Exception(curl_error($curl) ?: 'Empty response', curl_errno($curl));
        curl_close($curl);
        throw $exception;
    }
    curl_close($curl);
    return $response;
}

在本地它工作正常，但在实时服务器上我检索NULL。当我将jsonurl放入浏览器时，它返回有效的json。那么它可能与fetchUrl函数和curl_setopt_array有关吗？

请帮助我疯狂！