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<?php
session_start();
?>
<!DOCTYPE html>
...
<?php
require('fbsdk/autoload.php');

use FacebookFacebookSession;
...

FacebookSession::setDefaultApplication('...', '...');

$helper = new FacebookRedirectLoginHelper( '...' );

try {
  $session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {
} catch( Exception $ex ) {
}

if ( isset( $session ) ) {
  $request = new FacebookRequest( $session, 'GET', '/mygroupid/members?limit=2000' );
  $response = $request->execute();

    $data = $response->getGraphObject();
    echo '<pre>', var_dump($data), '</pre>';

} else {
  echo '<a href="' . $helper->getLoginUrl() . '">Login</a>';
}

?>
</body>
</html>