This is based on the nifty webiopi guide here.
Install Apache and PHP
sudo apt-get install apache2 php5
Add Apache's user (www-data) to the sudoers if needed
This may be necessary if you want web pages to control IO pins etc. Note that by doing this you introduce the possibility of security vulnerabilities via apache. If you are using your RPi on a local network and this is not a concern then you may well not care about this but if you are using your RPi on a public network where security is a concern then you may want to skip this step and find an alternative solution if you hit security permission issues.
Add this line to the end of the file:
www-data ALL=(ALL) NOPASSWD: ALL
Press CTRL + X to save and exit.
sudo /etc/init.d/apache2 restart
If you type your Raspberry Pi IP address into the web browser on a machine connected to the network you should see the apache default web page appear.
Adding Your HTML, PHP etc Files
Copy them into the "/var/www/html/" directory.
You will need root permission to write to this directory, so if you are using the GUI file manager use "sudo su" to elevate yourself to the root user before starting the GUI with "startx"
sudo service apache2 restart
Change The Password!
If your RPi will be connected to the internet or a public network a priority is to make the Raspberry Pi more secure by not using the default username and password. To change the password for the pi user after logging in:
If you are not using the HDMI out it is sensible to reduce the amount memory given to the RPi graphics to say 16MB using the config tool:
The configuration file
sudo nano /etc/apache2/sites-enabled/000-default.conf