facebook_desktop.php

<?php
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// | Facebook Platform PHP5 client                                             |
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/**
 *  This class extends and modifies the "Facebook" class to better
 *  suit desktop apps.
 */
class FacebookDesktop extends Facebook {
  // the application secret, which differs from the session secret
  public $app_secret;
  public $verify_sig;

  public function __construct($api_key, $secret) {
    $this->app_secret = $secret;
    $this->verify_sig = false;
    parent::__construct($api_key, $secret);
  }

  public function do_get_session($auth_token) {
    $this->api_client->secret = $this->app_secret;
    $this->api_client->session_key = null;
    $session_info = parent::do_get_session($auth_token);
    if (!empty($session_info['secret'])) {
      // store the session secret
      $this->set_session_secret($session_info['secret']);
    }
    return $session_info;
  }

  public function set_session_secret($session_secret) {
    $this->secret = $session_secret;
    $this->api_client->secret = $session_secret;
  }

  public function require_login() {
    if ($this->get_loggedin_user()) {
      try {
        // try a session-based API call to ensure that we have the correct
        // session secret
        $user = $this->api_client->users_getLoggedInUser();

        // now that we have a valid session secret, verify the signature
        $this->verify_sig = true;
        if ($this->validate_fb_params(false)) {
          return $user;
        } else {
          // validation failed
          return null;
        }
      } catch (FacebookRestClientException $ex) {
        if (isset($_GET['auth_token'])) {
          // if we have an auth_token, use it to establish a session
          $session_info = $this->do_get_session($_GET['auth_token']);
          if ($session_info) {
            return $session_info['uid'];
          }
        }
      }
    }
    // if we get here, we need to redirect the user to log in
    $this->redirect($this->get_login_url(self::current_url(), $this->in_fb_canvas()));
  }

  public function verify_signature($fb_params, $expected_sig) {
    // we don't want to verify the signature until we have a valid
    // session secret
    if ($this->verify_sig) {
      return parent::verify_signature($fb_params, $expected_sig);
    } else {
      return true;
    }
  }
}