First you get a token for the broadcaster, like you did for logging a user in.
Then you make a call to the subscribers API the same way you are already making calls to the users API
you take
$curl = curl_init('https://api.twitch.tv/helix/user');
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HTTPHEADER, array(
'Client-ID: ' . $client_id,
'Authorization: Bearer ' . $result['access_token']
));
curl_setopt($curl, CURLOPT_TIMEOUT, 1000);
$user = curl_exec($curl);
$i = curl_getinfo($curl);
curl_close($curl);
and substitute the variables
$curl = curl_init('https://api.twitch.tv/helix/subscriptions?broadcaster_id=' . $broadcaster_id . '&user_id' . $user_id);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HTTPHEADER, array(
'Client-ID: ' . $client_id,
'Authorization: Bearer ' . $broadcasters_token
));
curl_setopt($curl, CURLOPT_TIMEOUT, 1000);
$subdata = curl_exec($curl);
$i = curl_getinfo($curl);
curl_close($curl);