How to use replacement variables?

[Peter Ostry]

I posted this on vbulletin.com but maybe here is a better place.
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In the registration "welcome" mail I need the URL of the server as a clickable link. Not the URL of the forum, which is $vboptions[bburl] but the servername alone. I made a replacement variable called 'serverurl' that contains the PHP code for the server name:

http://<? echo $_Server['SERVER_NAME']; ?>

The vBulletin manual talks about advantages and dangers of replacement variables but I cannot find an example how to actually use them. I tried my replacement variable in the phrase of the welcome mail in various flavours:
[serverurl]
$vboptions[serverurl]
<serverurl>
etc, nothing worked.

How do I call a replacement variable?
Can I have PHP code in a replacement variable?
Is there another method to get just the URL of the server?