The Apache module mod_vhost_alias and its VirtualDocumentRoot directive can really be a great time saver for local development (some googling will explain why in more deapth). Basically, my local dev is set up so that I just have to create a directory in my aliases directory, and I just then navigate my browser to a URL matching the name of that new directory, and apache knows exactly what to serve automagically.
However, there are a few evil gotchas when using mod_vhost_alias, one of which is that the PHP global $_SERVER['DOCUMENT_ROOT'] remains set to the apache default DOCUMENT_ROOT environment variable rather than being re-assigned to the document root activated by the VirtualDocumentRoot directive for the current URL. This can cause some PHP applications (that are too trusting) to die for one reason or another.
I found a great solution to this in the related apache bug report: Simply add the following line to your apache configuration inside the VirtualDocumentRoot vhost definition:
php_admin_value auto_prepend_file /path/setdocroot.php
Then, create the referenced PHP file, and put set this as its contents:
<?php $_SERVER['DOCUMENT_ROOT'] = str_replace($_SERVER['SCRIPT_NAME'], '', $_SERVER['SCRIPT_FILENAME']);
Now, every page load has this file executed, which properly sets DOCUMENT_ROOT.