In the past I've always had to make two entries for every new site I add to my local development environment:

1. a line in my hosts file (so the site gets served locally)
2. a VirtualHost directive in my vhosts file (to tell apache where to find the web files)

This quickly becomes unsustainable. How would it be if I could just add a new folder to my apache web root and immediately load the new site in my browser? The following setup (adapted from Glen Scott) makes that possible!

This setup uses the subdomain "local" for all locally developed sites. So if I'm working on a site called "blah.com", my local version would be at "local.blah.com". Here's how you do it:

1. Edit your httpd.conf file

You need to enable two things to make this work:

1. mod_vhost_alias
2. httpd-vhosts.conf

Find the lines with these directives and uncomment them (your paths may differ):

#LoadModule vhost_alias_module libexec/apache2/mod_vhost_alias.so
...
#Include /private/etc/apache2/extra/httpd-vhosts.conf

2. Edit your httpd-vhosts.conf file

Paste in this block (adjust the path to your apache web root as necessary):

# This uses the requested hostname to map the file path to the web documents to
# be served. I don't want to have to use the "local" subdomain prefix on all my
# website folders, so the VirtualDocumentRoot will start at the second dot-
# section of the hostname and use everything from that point forward. So a
# request to "local.domain.com" will be served from the folder "domain.com" in
# my apache web root.
<VirtualHost *:80>
    ServerName local.*
    ServerAlias local.*
    UseCanonicalName Off
    VirtualDocumentRoot "~/Sites/%2+"
    <Directory "~/Sites/%2+">
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        RewriteEngine on
    </Directory>
</VirtualHost>

3. Fix your DOCUMENT_ROOT

One side-effect of using mod_vhost_alias is that your DOCUMENT_ROOT defaults to the apache web root, not the document root of the virtual host being served. So if you use the PHP global $_SERVER['DOCUMENT_ROOT'] in your code, it will contain the wrong value. To fix this problem, we'll tell PHP to require a file that correctly defines our document root before running any PHP scripts.

Create the following file and save it somewhere that makes sense to you. For example at: ~/Sites/_config/set-doc-root.php.

<?php
/**
 * Sets the correct DOCUMENT_ROOT value for php
 *
 * Using the apache module mod_vhost_alias to map the path to virtual host
 * document roots has the unfortunate side-effect that the PHP global variable
 * $_SERVER['DOCUMENT_ROOT'] is not set correctly. It defaults to your apache
 * web root, not the document root of the virtual host being served. This
 * script should fix that problem.
 *
 * USAGE:
 * In your php.ini file, add the path to this script in the
 * "auto_prepend_file" directive. Then it will be prepended to every PHP
 * script that runs.
 *
 * NOTE: The file must exist and be readable by apache or else you get a fatal
 * error because it gets included as if called by require().
 */

$_SERVER['DOCUMENT_ROOT'] = str_replace($_SERVER['SCRIPT_NAME'], '', $_SERVER['SCRIPT_FILENAME']);

4. Create a PAC file

We need to make sure that any requests with the "local" subdomain ("local.anything") get routed to our local apache server. To do this we'll use a proxy auto-configuration (PAC) file.

Create the following PAC file:

function FindProxyForURL(url, host)
{
    // if the host is "localhost" or starts with "local.", then serve it locally
    if (host == "localhost" || shExpMatch(host, "local.*")) {
       return "PROXY localhost";
    }
    // otherwise go to the internet
    return "DIRECT";
}

Save the file with a ".pac" extension somewhere in your apache web root (it must be accesible to apache). For example at: ~/Sites/_config/local.pac.

5. Tell apache to use your PAC file

Now we need to adjust our System Preferences in OS X to tell apache to use our proxy auto-configuration file.

1. Go to System Preferences > Network.
2. Select your network in the left-hand pane and click the "Advanced" button in the lower-right.
3. Click the "Proxies" tab and check "Automatic Proxy Configuration".
4. In the "URL" field enter the URL to your PAC file from above. For example: "http://localhost/_config/local.pac"

Now resart apache to apply the settings.

sudo apachectl restart

6. Test it

Now you should be able to add a folder to your apache web root and immediately see it in your browser. Try it out!

mkdir -p ~/Sites/newdomain.com
echo 'Hello World!' > ~/Sites/newdomain.com/index.html

Now visit http://local.newdomain.com/ in your browser and you should see "Hello World!".

That's it!