Fetch number of members in a Slack channel

slack_members.php

<?php
// Your ID and token
$botToken = 'xoxb-xxxxxxxxxx-xxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxx';
$channelID = 'C12345678';

// Setup cURL
$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => "https://slack.com/api/conversations.info?channel=$channelID&include_num_members=true",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_CUSTOMREQUEST => 'GET',
  CURLOPT_HTTPHEADER => array(
    'Content-Type: application/x-www-form-urlencoded',
    "Authorization: Bearer $botToken"
  ),
));

$response = curl_exec($curl);
$responseData = json_decode($response, TRUE);
curl_close($curl);

echo $responseData['num_members'];