For some reason my MythWeb interface just up and disappeared a few days ago. I went to the place the web page was supposed to be, but I got a blank page. The machine served other PHP pages just fine. After poking around I found this ubuntu forum post. I just followed these instructions:
sudo apt-get remove –purge php* apache* mythweb*
sudo rm -rf /etc/apache2
sudo apt-get install apache2-dev php5-dev libapache2-mod-php5 apache2-doc mythweb
I also had to reinstall the mythtv-backend package.
Basically what it did was uninstall Apache, PHP, and MythWeb completely (also Mythtv-backend) and reinstalled them.