为什么我的accestoken是null啊，又没报错？老师，这是怎么回事儿

//获取accesstoken

function getWxAccessToken(){

//1.请求url地址

$appid = 'wx5e0aeb45ca4c889d';

$appsecret =  'a925010ccbfb2b8323c98068e9951a49';

$url = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=".$appid."&secret=".$appsecret;

//2初始化

$ch = curl_init();

//3.设置参数

curl_setopt($ch , CURLOPT_URL, $url);

curl_setopt($ch , CURLOPT_RETURNTRANSFER, 1);

//4.调用接口 

$res = curl_exec($ch);

//5.关闭curl

curl_close( $ch );

if( curl_errno($ch) ){

var_dump( curl_error($ch) );

}

$arr = json_decode($res, true);

var_dump( $arr );

}