As detailed on Hivelogic, the software update to Mac OS X 10.2.4 overwrites /etc/conf/httpd.conf, so if you use the OS X Apache installation for local hosting and debugging, you need to restore your Apache configuration from the backup that the update is kind enough to make. For me, this involved pasting my old VirtualHosts into the new conf file, uncommenting the lines which enable PHP, and adding the line AddType application/x-httpd-php4. Simple as that.
On a brigher note, it looks like Safari Beta v62 is going to have tabs. And there was much rejoicing.