Pointing a Sub-domain to a sub directory - php

Question

Using Laravel I'm generating php pages in to a sub-directory called webpage. I want to allow users to access pages using sub-domain. For example, I've generated a file whoami.php into webpage directory. If people try to access whoami.domain.com, he can see the whoami.php. I've crated an index.php inside webpage which can extract alias and show the file. The think I need is to pass the subdomain to index.php file without redirecting. I wrote htaccess as follow
<IfModule mod_rewrite.c>
<IfModule mod_negotiation.c>
Options -MultiViews
</IfModule>
RewriteEngine On
# Redirect Trailing Slashes...
RewriteRule ^(.*)/$ /$1 [L,R=301]
# Handle Front Controller...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^ index.php [L]
#sub domain custom part
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^(.*)\.domain\.com
RewriteRule ^(.*)$ http://domain.com/webpage/%1/$1 [L,NC,QSA]
</IfModule>
But I'm redirecting into http://domain.com/webpage/whoami.
I tried to make my requirements clearly and pasted what I tried. Can you please help me to figure out a way?
EDIT 1
Based on #Starkeen's answer I tried the following code an now in a redirect loop (http://whoami.domain.com/webpage/whoami/webpage/whoami/......).
<IfModule mod_rewrite.c>
<IfModule mod_negotiation.c>
Options -MultiViews
</IfModule>
RewriteEngine On
#sub domain
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^(.*)\.domain\.com$
RewriteRule ^(.*)$ /webpage/%1/$1 [L,NC,R]
# Redirect Trailing Slashes...
RewriteRule ^(.*)/$ /$1 [L,R=301]
# Handle Front Controller...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^ index.php [L]
</IfModule>

Answer 1

To redirect subdomain to a subfolder, You probably need to put the following right after the "RewriteEngine On" line :
RewriteCond %{HTTP_HOST} ^(sub)\.domain\.com$
RewriteRule ^(.*)$ /webpage/%1/$1 [NC,R,L]

Answer 2

I got a every simple solution. As I'm using Laravel and that already use an index.php as bootstrap, I wrote some PHP codes there to achieve my goal. You guys can use if needed. And please advice me if my solution isn't good enough. Thanks
My Solution
$link = "http://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";
$link = str_replace(array('http://', 'https://', 'www.'), '', $link);
$splitted = explode(".", $link);
if($splitted[0] != 'domain'){
$file_name = $splitted[0].'.php';
$file_loc = 'webpage/'.$file_name;
if(file_exists($file_loc)) {
include($file_loc);
} else {
header('Location: http://domain.com/not-found');
}
die();
}