Suppose you have many links leading to a site like:

thelongestdomainnameintheworldever.com

so instead of typing this on all your <a href>'s you type something like hi.com. Can you tell your htaccess file that every time hi.com appears on your website that it should actually link to:

thelongestdomainnameintheworldever.com

What would be the actual htacess code for this?

Assuming that you have access to hi.com to change the htaccess file there, you need to do a 301 redirect. How exactly you do it depends on a lot of things, like do both domains point to the same site? Are you just redirecting one page or all pages? Does hi.com/foo.html get redirected to reallylongurl.com/foo.html or something else?

There's tons of tutorials on 301 redirects out there, this one looks decent:
http://www.isitebuild.com/301-redirect.htm

I think you are misunderstanding the purpose of .htaccess, but I cannot be sure without a more concrete explaination of what you are trying to do. If you are just trying to shorten your URLs I don't recommend using redirects for this. You could accomplish this with a simple rewrite rule, but having all of your internal links converted to external links which get redirected to an internal page seems like a bad approach to me.

If your domain name is seriously too long to type out in your code, you can do something like this:

PHP Code:

<a href="<?php $_SERVER['HTTP_HOST']; ?>/index.php">My Link</a>

or

PHP Code:

<?php
$url = $_SERVER['HTTP_HOST'];
?>
<a href="<?php echo $url; ?>/index.php">My Link</a>

Not an ideal situation, but I really advise against using another domain and redirection.

Or... why are you using complete urls at all??? If you need to use absolute file pathing, then simply do

/directory/file.file

This will go to http://www.thelongestdomainnamever.c...tory/file.file

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^www.short-example.com$ [NC]
RewriteRule ^(.*)$ http://www.long-example.com/$1 [L,R=301]