返回响应字符串以设置Webhook

I'm trying to integrate PHP Webhooks with the Onfleet API. The Documentation says I must return a simple string response

http://docs.onfleet.com/docs/webhooks#create-webhook

Here is my code

$input = file_get_contents("php://input");

if (isset($input)) {
    header('Content-Type: application/json');
    echo $input->check;
}

I keep receiving the error "Value received in response does not match the value we sent you"

I would do it like this

if ($_SERVER['REQUEST_METHOD'] === 'GET' && isset($_GET['check']))
{
    header('Content-Type: application/json');
    echo $_GET['check'];
}