curl_exec($ch)的返回值一直是false，调用curl_errno($ch)是空的？？？

public function http_curl($url,$type='get',$res='json',$arr=''){
    // 初始化
    $ch=curl_init();
    // 设置参数
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch,CURLOPT_HEADER,FALSE);
    if($type == 'post'){
        curl_setopt($ch,CURLOPT_POST, true);
        curl_setopt($ch,CURLOPT_POSTFIELDS,$arr);
    }
    $output=curl_exec($ch);
    curl_close($ch);
    var_dump($output);
    echo 'error:'.curl_errno($ch);
    if(curl_errno($ch)){
        var_dump(curl_errno($ch));
    }
    if($res == 'json'){
        return json_decode($output,true); 
    }
        return $output;
}