Giving WordPress Its Own Directory together with index.html

Here is the code to put in your functions.php file, it will show the splash-page.php only to outside visitors (by checking the referer). Make a splash-page.php in your current theme directory and edit the domain (example.com) in the code:

add_action('template_redirect','my_splash_page');      
function my_splash_page(){ 
    $referer = $_SERVER['HTTP_REFERER'];
    $referer_parse = parse_url($referer);
    if(is_front_page()){
        // let visitors see the normal wordpress homepage if they are browsing your web
        if($referer_parse['host'] == "example.com" || $referer_parse['host'] == "www.example.com") {

        // let visitors see the splash page if they are visiting directly or from another pages
        } else {
            include(TEMPLATEPATH.'/splash-page.php');
            exit();
        }
    }
}