You’ve been given the relevant information, samples and explanation on how the flow.
We cannot do all your work for you.
Get and store a key from the broadcaster with the channel:read:subscriptions and refresh said key as needed
Then a regular user will Login to your website
Then use the broadcasters key with the logged in user ID against the broadcaster subscriptions API to check status
It’s not a single script, it’s two web pages/flows and a cronjob to maintain the broadcasters key.
You can even modify the user authentication script I already gave you to get the broadcasters key, you just need to determine where you want to store the broadcasters key for reuse