读写文件发生错误：Too many open files，如何解决？

ubuntu系统
Warning: file_put_contents(/xxxx/log//17-03-22.txt): failed to open stream: Too many open files in /xxx/common/function.php on line 323

这是方法是写日志，用的是file_put_contents方法，按理说写完日志就关掉了文件连接了，而我的work就看了3个进程，就导致了这个问题，请问如何解决？

调试发现是curl_init造成的，但是有关闭，系统却一直保持连接

protected function _wget($url, $post_data = null)
    {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_BINARYTRANSFER, true);
        curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
        curl_setopt($ch, CURLOPT_TIMEOUT, 15);
        if ($post_data) {
            curl_setopt($ch, CURLOPT_POST, 1);
            curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);
        }
        $response = curl_exec($ch);
        if ($response === false) {
            system_log('获取远程数据失败：' . curl_error($ch));
            curl_close($ch);
            return false;
        }
        $http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
        curl_close($ch);
        if ($http_code == 200) {
            $ret = json_decode($response, true);
            if ($ret) {
                return $ret;
            }
            system_log('获取远程数据返回异常内容:' . $response);
        } else {
            system_log('获取远程数据返回异常状态码:' . $http_code . ',返回内容为:' . $response);
            return false;
        }
    }