View Source and View Generated Source in firefox addons Web Developer

I use Firefox Web developer everyday.

Now I found some of my website need to use View Generated Source rather than View Source.

Because View Source gives a blank outputs.

It seems it is relating to the following code.

I am hoping someone can tell me why it needs View Generated source.

And is there anyway I can开发者_如何学Go modify the code so that I don't need to worry about it.

Or I don't need to worry about it?

I use PHP/MySQL.

 if (!isset($_SESSION['userid']) || $_SESSION['userid'] < 1){
    redirect('welcome/verify','refresh');
}

You might consider using Firebug. It will allow you to see the rendered HTML as well as browse the DOM and see what is sent to and from the browser in great detail.