app_token.php

  $ch = curl_init('https://www.arcgis.com/sharing/oauth2/token');
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
  curl_setopt($ch, CURLOPT_POST, TRUE);
  curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array(
    'grant_type' => 'client_credentials',
    'client_id' => $client_id,
    'client_secret' => $client_secret
  )));
  $response = json_decode(curl_exec($ch));
  
  $access_token = $response->access_token;

  echo '<pre>';
  print_r($response);