My domain is meeting.com.

Sometimes with PHP, jQuery, for that primary part. No ASP.

Whenever a user produces a merchant account, I would like a subdomain user.meeting.com to become produced for REDIRECTION reasons only. I'm not going the subdomain to exists for real.

When someone would load john13.meeting.com, I'd like the web site to redirect to meeting.com?user=john13 (I do not care if john13.meeting.com remains within the URL bar).

What's the most effective and simple method it instantly in PHP or Apache? Please consider performance too since i have have numerous customers.

You do not need PHP for your. The best Apache configuration using the rewrite module on, will have the desired effect.

For instance (not examined):

RewriteCond %{HTTP_HOST} !^www\.meeting\.com?$
RewriteCond %{HTTP_HOST} ^([^.]+)\.meeting\.com?$
RewriteRule ^$ /user/%1 [L]

Or maybe you are not spinning further for Search engine optimization, the rule might be

RewriteRule ^$ /index.php?user=%1 [L]

For additional inspiration, browse the link Shef published with another example of methods to do this.