Enabling PHP

PHP is installed in Mac OS X by default, but not enabled. To enable it, we must edit the apache 2 configuration file, which is located at /etc/apache2/httpd.conf. Find the line which loads the PHP 5 module, which looks like this:

#LoadModule php5_module libexec/apache2/libphp5.so

The line is currently commented out. All we have to do is remove the comment symbol, #, so the line looks like this:

LoadModule php5_module libexec/apache2/libphp5.so

Save.

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Enabling a Personal Website

If you clicked on the URL under "Your Personal Website," you might have gotten a page that says forbidden. This is because in the default configuration in Leopard, unlike in Tiger, does not allow Apache to serve documents from home directories. If you want to enable this feature, you have to create a new Configuration file.
Create a new file with the following contents and save it to /etc/apache2/users/johndoe.conf.

<directory "/Users/johndoe/Sites">
Options Indexes MultiViews FollowSymLinks
AllowOverride All
Order allow,deny
Allow from all
</directory>

Replace "jeff" with your user name, which is also the name of your home directory. Exact capitalization is imporant. This tells the Apache server that it is ok to serve web content out of the ~jeff directory. You will have to restart Apache for this to take effect.
You may also have to create a Sites folder in your home directory to hold the files you want to serve. Leopard will automatically bless this folder with a special Icon.

Info found on Professional PHP