php类 方法里调用curl（）,不成功

class Qq_api {

    private $list;

    function __construct() {
        $this->list['dsp_id'] = DSP_ID;
        $this->list['token'] = TOKEN;
    }

    function curl_post($end = '') {
$ch = curl_init(URL.$end);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    }

    function sync() {
        $this->curl_post('sync');


        $arr = array();



curl_setopt($ch, CURLOPT_POSTFIELDS,$arr);
$result = curl_exec($ch);
var_dump($result);


    }   
}

提示：Warning: curl_setopt(): supplied argument is not a valid cURL handle resource in C:\AppServ\www\qq_api2.php on line 45

我是在方法里调用curl（），好像curl（）只能在_construct（）里面初始化吧，如果是为什么？