The way to do it with the php extension intact would be:
www.something.com/user.php?id=1
You then use:
$userID = $_GET['id'];
If you want to drop the PHP extension, you can do this with a rewrite in the htaccess: This is pretty generic as it depends on your web-server, but something like:
RewriteRule ^www\.something\.com/user\.php$ /www.something.com/user?&%{QUERY_STRING}
Or you could just drop using the PHP extension on all of the files to keep it neat using something like:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^.]+)$ $1.php [NC,L]
This will rewrite all of your php extensions so this:
something.com/someFileName
Will automagically be read as
something.com/someFileName.php
To which you can still use:
something.com/someFileName?someQueryParameter=someValue