A request for account_credentials requires an account_id in the URL, and a ClientID/ClientSecret in the request itself: " To use account credentials to get an access token for your app, call the Zoom OAuth token API with the account_credentials grant_type , your account_id , and your client ID and client secret, base64 encoded (encode your client ID and client secret with a colon between them, e.g. client_id:client_secret )."
By the way, it’s far easier to create/compose these requests using Postman, or if you’re sold on just PHP, something like Guzzle.