A splash page on landing page

I would propose first to make this at the front end. The reason is Google and other crawlers can read your content and don’t penalize you for sneaky (the user didn’t expect a redirect) redirects or slow loading times.

What I would do:
On the main page :

function load_splash() {
 setTimeout(function () {
   $('#hiddendiv').hide();
 }, 5000);
}

Now the load_splash can be called when the document ready

$( document ).ready(function() {
   load_splash( );
});