foo.com/alice vs. foo.com/users/alice - url

Question

It's of course nice to give users friendly URLs for their content on your site. But how best to do that? There are a lot of advantages to something like foo.com/users/alice, most importantly that you aren't cluttering up your root namespace. But I think simplicity for users trumps all that. A lot of big sites seem to agree (friendfeed, delicious, and flickr come to mind) and this question is about how to accomplish that on the server side.
Let's assume the real URL for alice is foo.com/userpage?user=alice and that if someone tries to surf to a nonexistent user page (let's say foo.com/bob) they should reach foo.com/createnew?user=bob.
The user of course should never see the ugly "real" URLs above, just foo.com/alice or foo.com/bob. And note that the root namespace is shared. For example, foo.com/help should not get translated to foo.com/userpage?user=help.
Presumably I'm asking for some simple mod_rewrite rules, but perhaps there's some completely different approach to this that I'm not thinking of. In any case, I thought it would be good to record a definitive or "best practice" solution to this common question.
PS: Feel free to comment on the merits of other alternatives like alice.foo.com or users.foo.com/alice.
PPS: I think I've seen this issue debated in other questions but it seems to be tricky to search for. Pointers welcome! As well as additional keywords to make this more searchable, of course. Keywords: userspace, global namespace, URL namespace.

Answer 1

I would say it depends on how user centred your site is.
Sites like myspace are http://www.myspace.com/jim/ because the site entirely revolves around the user.
A blog or news site, however, where you can register but it isn't important or mandatory could benefit from
http://www.news.com.au/users/jim/
Do you think if you're doing a website with users you could benefit from the MVC design pattern, or at least a popular MVC framework which uses a router to direct URIs?
If that URI came through a Router, and then was sent to the UsersController, you could decide to either show the user's profile, or direct them to create that user. You would not need to mess around with mod_rewrite except to make one rule that directs all requests to non existent files to index.php (or whatever the default of your server side language is)
If you do want to use mod_rewrite, try these rules
RewriteEngine On
RewriteCond %{REQUEST_URI} !(home|contact|about) [NC] // this line may be incorrect
RewriteRule ^/users/([^/]+)/?$ userpage?user=$1 [NC,L]
Please note the leading Carat as suggested by Gumbo, so it only matches /users/ of the TLD only.
That will match anything like foo.com/users/bob with an optional trailing slash. It is case insensitive and will be the last rule applied.
If the request comes in and the $_GET['user'] does not exist in your DB, you could try something like this
$user = $_GET['user'];
if (!user_exists($user)) {
header('Location: createnew?user=' . urlencode($user));
exit();
}
Then on the createnew page, simply do something like this
<input type="text" name="username" value="<?php echo htmlspecialchars(urldecode($_GET['user'])); ?>" />
That will fill in the username automatically with the username they tried to access a profile with.
If you'd like to know more about PHP and MVC, try a Google search or ask a question here on Stack Overflow.

Answer 2

The following rules rewrite a URL of the form foo.com/bar to foo.com/userpage?user=bar conditional on bar not already being a file or directory on the server. Put the following in .htaccess:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/?$ userpage?user=$1 [NC,L]
</IfModule>
As in Alex's answer, the userpage script should redirect to createnew if the user doesn't exist:
$user = $_GET['user'];
if (!user_exists($user)) {
header('Location: createnew?user=' . urlencode($user));
}
(As Knuth says, beware of bugs in the above code -- I have only proved it correct, not tried it. I'll update this answer when I've actually confirmed it works.) PS: CONFIRMED!