Php 为什么curl返回空字符串

我正在尝试通过php，curl登录到一个网页。问题是curl返回一个空字符串，我不明白为什么

我发送的参数是正确的，我已经用Burpusuite检查过了

来自CURLOPT_STDERR的日志

我的代码：

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://gameforge.com/api/v1/auth/thin/sessions");
curl_setopt($ch, CURLOPT_POSTFIELDS, '{"identity":"'.$mail.'","password":"'.$password.'","locale":"es_ES",
    "gfLang":"es","platformGameId":"07b3e4d5-f37e-440b-9aa5-b249121a6bfa","gameEnvironmentId":"5c71b04b-a0c3-4792-be4f-81f97c25d8a8","autoGameAccountCreation":false}');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$res = curl_exec($ch);
print_r(curl_getinfo($ch));
curl_close($ch);
var_dump($res);
die();

打印结果（curl_getinfo（$ch））；和var_dump（$res）

---

阅读你的协议。您正在使用内容类型application/x-www-form-urlencoded，但正在发送application/json。

Array
(
    [url] => https://gameforge.com/api/v1/auth/thin/sessions
    [content_type] => 
    [http_code] => 409
    [header_size] => 666
    [request_size] => 146
    [filetime] => -1
    [ssl_verify_result] => 0
    [redirect_count] => 0
    [total_time] => 0.102569
    [namelookup_time] => 0.000876
    [connect_time] => 0.005851
    [pretransfer_time] => 0.023868
    [size_upload] => 253
    [size_download] => 0
    [speed_download] => 0
    [speed_upload] => 2480
    [download_content_length] => 0
    [upload_content_length] => 253
    [starttransfer_time] => 0.023873
    [redirect_time] => 0
    [redirect_url] => 
    [primary_ip] => 121.23.13.2
    [certinfo] => Array
        (
        )

    [primary_port] => 443
    [local_ip] => 31.23.93.11
    [local_port] => 39462
    [http_version] => 3
    [protocol] => 2
    [ssl_verifyresult] => 0
    [scheme] => HTTPS
    [appconnect_time_us] => 23672
    [connect_time_us] => 5851
    [namelookup_time_us] => 876
    [pretransfer_time_us] => 23868
    [redirect_time_us] => 0
    [starttransfer_time_us] => 23873
    [total_time_us] => 102569
)
string(0) ""