curl 调用支付接口带数据提示 403 不带数据可以访问

我用curl调用柬埔寨wing支接口如果带上数据报错

但是如果不带数据就能调用

请问是什么原因？？？
这是我curl方法的代码：

function wing_pay($amount = 1,$return_url = '',$remark = '')
{
    $url = 'https://stageonline.wingmoney.com/wingonlinesdk/';
    $post_data = '';
    $curl = curl_init();
    curl_setopt_array($curl, array(
        CURLOPT_URL => $url,
        CURLOPT_REFERER => 'http://'.$_SERVER['HTTP_HOST'],
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_SSL_VERIFYPEER => false,
        CURLOPT_SSL_VERIFYHOST => false,
        CURLOPT_AUTOREFERER => true,
        CURLOPT_FAILONERROR => false,
        CURLOPT_FOLLOWLOCATION => true,
        CURLOPT_MAXREDIRS => 10,
        CURLOPT_TIMEOUT => 30,
        CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
        CURLOPT_CUSTOMREQUEST => "POST",
        CURLOPT_POSTFIELDS => $post_data,
        CURLOPT_HTTPHEADER => array("cache-control: no-cache", "content-type: application/x-www-form-urlencoded"),));
    $response = curl_exec($curl);
    $err = curl_error($curl);
    curl_close($curl);
    if ($err) {
        echo "cURL Error #:" . $err;
    } else {
        echo $response;
    }
}