Error registering webhook



I am trying to register my webhook doing a post request to account_activity/webhooks but I am getting “Webhook URL does not meet the requirements.” error.

For crc I am doing this in php:

$data = file_get_contents("php://input");
$code= hash_hmac("sha256", $data_decoded['crc_token'] , $this->consumer_secret);
$response = array(
			'response_token'=> 'sha256='.base64_encode($code)
header('Content-Type: application/json');
header('HTTP 200 ok');
echo json_encode($response);

My webhook answers in less than 1 second and the response token is correct, so I don’t know why is not working. Can someone help me?