(Updated) Laravel PUT Method Not Working - php

Question

updated - the JSON file would return but it will not change the billing date at all.
Reference 1: The official documentation about changing the billing date.
Reference2: their sample code in detail:
<?php
$request = new HttpRequest();
$request->setUrl('https://domain.chargify.com/subscriptions/$subscriptionId.json');
$request->setMethod(HTTP_METH_PUT);
$request->setHeaders(array(
'authorization' => 'Basic YXBpa2V5Og==',
'content-type' => 'application/json'
));
$request->setBody('{"subscription":{"next_billing_at":"2028-12-15"}}');
try {
$response = $request->send();
echo $response->getBody();
} catch (HttpException $ex) {
echo $ex;
}
My code in detail:
public function changeYearlySubscriptionBillingDate(Request $request)
{
$user = $request->user();
$subscriptionId = $user->subscription->subscription_id;
$nextBilling = Carbon::now()->addYear();
$hostname = env('CHARGIFY_HOSTNAME');
$headers = [
'authorization' => 'Basic ANIDIANDIAJIJCQ',
'content-type' => 'application/json'
];
$body = ["subscription" => ["next_billing_at" =>[ $nextBilling ]]];
$config = [
'headers' => $headers,
'form_param' => $body
];
$client = new Client($config);
$res = $client->put("https://$hostname/subscriptions/$subscriptionId.json",
["json" => [
[ "subscription" =>
[ "next_billing_at" => $nextBilling ]
]
]
]);
echo $res->getBody();
}

Answer 1

The url you are building is incorrect. There shouldn't be a / between $subscription and .json
Change
$res = $client->put("https://$hostname/subscriptions/$subscriptionId/.json",
to
$res = $client->put("https://$hostname/subscriptions/$subscriptionId.json",
EDIT
Try changing the logic to make Guzzle call like this.
$headers = [
'authorization' => 'Basic ANIDIANDIAJIJCQ',
'content-type' => 'application/json'
];
$body = ["subscription" => ["next_billing_at" =>[ $nextBilling ]]];
$client = new Client();
$res = $client->put("https://$hostname/subscriptions/$subscriptionId.json",
[
'headers' => $headers,
'body' => json_encode($body)
]
);
echo $res->getBody()->getContents();

Answer 2

thanks everyone so much for helping me out.
I've been working on this problem for 2 days and it was supposed to be right to me. Eventually, it is their API that misled me.
the only thing we need to do, is simply change
'body' => "{\"subscription\":{\"next_billing_at\":\"$nextBilling\"}}"
added a few '\' s inside.
Thanks everyone for helping me out, have a good day!