Get page access token from Facebook page

get_page_access_token.php

<?php

require_once __DIR__ . '/vendor/autoload.php';

use Facebook\Authentication\AccessToken;
use Facebook\FacebookApp;
use Facebook\FacebookRequest;

session_start();

$app_id = '{APP ID}';
$app_secret = '{APP SECRET}';

$app = new FacebookApp($app_id, $app_secret);

$fb = new Facebook\Facebook(array(
    'app_id' => $app_id,
    'app_secret' => $app_secret,
    'default_graph_version' => 'v2.5',
));

$page_id = '{PAGE ID}';

//User access token get from cakllback.php
$access_token = new AccessToken('{USER ACCESS TOKEN}');

$request = new FacebookRequest(
    $app, 
    $access_token, 
    'GET', '/' . $page_id,  
    array( 'fields' => 'access_token') );

// Send the request to Graph
try {
  $response = $fb->getClient()->sendRequest($request);

  $graphNode = $response->getGraphNode();

  echo 'Page Access Token: ' . $graphNode['access_token'];

} catch(Facebook\Exceptions\FacebookResponseException $e) {
  // When Graph returns an error
  echo 'Graph returned an error: ' . $e->getMessage();
  exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
  // When validation fails or other local issues
  echo 'Facebook SDK returned an error: ' . $e->getMessage();
  exit;
}