I am sending subscription request as follows

<form action="https://www.strava.com/api/v3/push_subscriptions" method="post" id="myform">
     <input type="hidden" name="client_id" value="XXX"/>
     <input type="hidden" name="client_secret" value="XXXX"/>
     <input type="hidden" name="callback_url" value="XXXX"/>
     <input type="hidden" name="verify_token" value="STRAVA"/>
     <input type="submit" value="Connect Webhook"/>
</form>

And then I am trying to validate the callback using PHP:

$hubMode = $_GET['hub_mode']; 
$hubVerifyToken = $_GET['hub_verify_token'];
$hubChal = $_GET['hub_challenge'];
$submitChallenge = array("hub.challenge"=>$hubChal);
header('Content-Type: application/json;');
http_response_code(200);
echo json_encode($submitChallenge);

I keep getting error as {"message":"Bad Request","errors":[{"resource":"PushSubscription","field":"callback url","code":"not verifiable"}]}

Please help on what I am doing wrong.