.htaccess Mod Rewrite code? - .htaccess

Question

I have searched for this but nobody have the same issue..
Now I need an code to transform the ugly url (thats what i have) to the nice url:
Ugly url: domain.com/aanmelden?referral=admin
Nice url: domain.com/aanmelden/admin
Ik have tried soo many codes, but no one did work for me. ):
suggestions?
Thnx!
(My current htaccess: )
ErrorDocument 401 /errordocs/401.php
ErrorDocument 403 /errordocs/403.php
ErrorDocument 404 /errordocs/404.php
ErrorDocument 500 /errordocs/500.php
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !^domain.nl$ [NC]
RewriteRule ^(.*)$ http://domain.nl/$1 [L,R=301]
RewriteCond %{REQUEST_URI} !/$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\.]+)$ $1.php [NC,L]
RewriteCond %{THE_REQUEST} ^[A-Z]+\s.+\.php\sHTTP/.+
RewriteRule ^(.+)\.php $1 [R=301,L]
Where and what i need to place?
EDIT:
To be clear:
If the user goes to the Nice Url, they wil stay on the nice one.
If the user goes to the Ugly Url, they wil redirect to the Nice url.

Answer 1

This is a trivial rewrite:
If admin can be any lowercase string, use the pattern ([a-z]+)$ to capture one or more lowercase letters after the / and before the end of the request URI.
RewriteEngine On
RewriteRule ^aanmelden/([a-z]+)$ aanmelden?referral=$1 [L]
If admin is really only admin, you can hard-code it as
RewriteEngine On
RewriteRule ^aanmelden/admin aanmelden?referral=admin [L]
Update
In context of your existing rewrites just posted, you'll need to add this rule before the rule that adds .php since that would also match this pattern.
# Do this before the rule that adds .php
# Also added condition so this doesn't apply to real files...
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^aanmelden/([a-z]+)$ aanmelden?referral=$1 [L]
RewriteCond %{REQUEST_URI} !/$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\.]+)$ $1.php [NC,L]
Update 2
If you want the end user to get the ugly URL and rewrite it to the nice one, use:
RewriteCond %{REQUEST_QUERYSTRING} referral=([a-z]+)
RewriteRule ^aanmelden aanmelden/%1 [L]