callback.php

<?php
  /**
   * Recebe o token e publica na page.
   * Facebook SDK version 2.5
   *
   * @author Thiago A. de Lima <thiagolimaes@gmail.com>
   */
  require_once 'Facebook/autoload.php';
  
  var $appId     = 'xxxxxx';
  var $appSecret = 'xxxxxx';
  var $pageId    = 'xxxxxx';
  
  $fb = new \Facebook\Facebook([
    'app_id'                => $appId,
    'app_secret'            => $appSecret,
    'default_graph_version' => 'v2.5'
  ]);
  
  $helper = $fb->getRedirectLoginHelper();
  
  try {
    $accessToken = $helper->getAccessToken();
  }
  catch (Facebook\Exceptions\FacebookResponseException $e) {
    echo 'Graph returned an error: ' . $e->getMessage();
    exit;
  }
  catch (Facebook\Exceptions\FacebookSDKException $e) {
    echo 'Facebook SDK returned an error: ' . $e->getMessage();
    exit;
  }
  
  if (!isset($accessToken)) {
    echo 'No OAuth data could be obtained from the signed request. User has not authorized your app yet.';
    exit;
  }
  
  /*
   * Para realizar a publicação na Page, é necessário capturar o token de acesso da page. 
   */
  function getPageToken($pageId, $accessToken){
    $uri      = 'https://graph.facebook.com/v2.5/'.$pageId.'?access_token='.$accessToken;
    $uri     .= '&fields=access_token&format=json&method=get&pretty=0&suppress_http_code=1';
    $response = json_decode(file_get_contents($uri));
    
    return $response->access_token;
  }
  $pageAccessToken = getPageToken($pageId, $accessToken);
  
  /* Monto os dados que serão publicados  */
  $data = array(
    'link'         => 'http://google.com',
    'message'      => 'Texto que será publicado!',
    'published'    => true,
    'access_token' => $pageAccessToken
  );
  
  /* Envio os dados para a page */
  try {
    $response = $fb->post('/'.$pageId.'/feed', $data, $pageAccessToken);
  }
  catch(Facebook\Exceptions\FacebookResponseException $e) {
    echo $e->getMessage();
  }
  catch(Facebook\Exceptions\FacebookSDKException $e) {
    echo $e->getMessage();
  }

login.php

<?php
  /**
   * Gera o token de acesso ao Facebook
   * Facebook SDK version 2.5
   *
   * @author Thiago A. de Lima <thiagolimaes@gmail.com>
   */
  require_once 'Facebook/autoload.php';
  
  var $appId       = 'xxx';
  var $appSecret   = 'xxxxxx';
  var $callbackURI = 'callback.php';
  
  $fb = new \Facebook\Facebook([
    'app_id'                => $appId,
    'app_secret'            => $appSecret,
    'default_graph_version' => 'v2.5'
  ]);
  
  $helper      = $fb->getRedirectLoginHelper();
  $permissions = ['manage_pages', 'publish_pages', 'publish_actions'];
  $loginUrl    = $helper->getLoginUrl($callbackURI, $permissions);
  
  header('Location: '.$loginUrl);