why it opened web and not app in my web when i use facebook login?

I use php in my app , but when the facebook login in my use ,it can opened web only ,can't open the app to authorization,why ?why ?why ?

$helper = $fb->getRedirectLoginHelper();
   $helper = $fb->getJavaScriptHelper();
   $helper = $fb->getCanvasHelper();
   $helper = $fb->getPageTabHelper();