Curl returns true instead of value

You need to echo not return:

echo $_POST['auth_token'];

And you need CURLOPT_RETURNTRANSFER to have curl_exec() return the content:

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

curl_exec

Returns TRUE on success or FALSE on failure. However, if the CURLOPT_RETURNTRANSFER option is set, it will return the result on success, FALSE on failure.