post-as-page.php

<?php
session_start();
require_once __DIR__ . '/src/Facebook/autoload.php'; // download official fb sdk for php @ https://github.com/facebook/php-graph-sdk

$fb = new Facebook\Facebook([
  'app_id' => 'APP_ID',
  'app_secret' => 'APP_SECRET',
  'default_graph_version' => 'v2.12',
  ]);

$helper = $fb->getRedirectLoginHelper();

$permissions = ['manage_pages', 'publish_pages']; // optional

try {
	if (isset($_SESSION['facebook_access_token'])) {
		$accessToken = $_SESSION['facebook_access_token'];
	} else {
  		$accessToken = $helper->getAccessToken();
	}
} 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;
 }

if (isset($accessToken)) {
	if (isset($_SESSION['facebook_access_token'])) {
		$fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
	} else {
		// getting short-lived access token
		$_SESSION['facebook_access_token'] = (string) $accessToken;

	  	// OAuth 2.0 client handler
		$oAuth2Client = $fb->getOAuth2Client();

		// Exchanges a short-lived access token for a long-lived one
		$longLivedAccessToken = $oAuth2Client->getLongLivedAccessToken($_SESSION['facebook_access_token']);

		$_SESSION['facebook_access_token'] = (string) $longLivedAccessToken;

		// setting default access token to be used in script
		$fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
	}

	// redirect the user back to the same page if it has "code" GET variable
	if (isset($_GET['code'])) {
		header('Location: ./');
	}

	// getting basic info about user
	try {
		$profile_request = $fb->get('/me');
		$profile = $profile_request->getGraphNode()->asArray();
	} catch(Facebook\Exceptions\FacebookResponseException $e) {
		// When Graph returns an error
		echo 'Graph returned an error: ' . $e->getMessage();
		session_destroy();
		// redirecting user back to app login page
		header("Location: ./");
		exit;
	} catch(Facebook\Exceptions\FacebookSDKException $e) {
		// When validation fails or other local issues
		echo 'Facebook SDK returned an error: ' . $e->getMessage();
		exit;
	}

	// post on behalf of page
	$pages = $fb->get('/me/accounts');
	$pages = $pages->getGraphEdge()->asArray();

	foreach ($pages as $key) {
		if ($key['name'] == 'Funny Demons') {
			$post = $fb->post('/' . $key['id'] . '/feed', array('message' => 'just for testing...'), $key['access_token']);
			$post = $post->getGraphNode()->asArray();
			print_r($post);
		}
	}

  	// Now you can redirect to another page and use the access token from $_SESSION['facebook_access_token']
} else {
	// replace your website URL same as added in the developers.facebook.com/apps e.g. if you used http instead of https and you used non-www version or www version of your website then you must add the same here
	$loginUrl = $helper->getLoginUrl('http://sohaibilyas.com/APP_DIR/', $permissions);
	echo '<a href="' . $loginUrl . '">Log in with Facebook!</a>';
}