How to use webhook for checking users are livestreaming and get there data?

API
14 
<?php

$client = '';
$token = '';
$user_id= '';
$callback = '';
$secret = '';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://api.twitch.tv/helix/eventsub/subscriptions');
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Client-ID: ' . $client,
    'Authorization: Bearer ' . $token
));
curl_setopt($ch, CURLOPT_POST, 1);

$payload = array(
    'type' => 'stream.online',
    'version' => '1',
    'condition' => array(
        'broadcaster_user_id' => $user_id
    ),
    'transport' => array(
        'method' => 'webhook',
        'callback' => $callback,
        'secret' => $secret
    )
);
print_r($payload);

curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($payload));

curl_exec($ch);

Successfully created a subscription request for me

Edit:

Earlier I copy/pasted your “broken header” but didn’t notice you put Accept where it should be Content-Type