Apache usually likes to be in /usr/local/apache but it doesn't have to be. PHP, if you install it as a module (Highly recommended) will place a file called libphp4.so into the /libexec folder under where Apache is installed. There is almost nothing that you have to do once you've installed PHP to get it to work, but the installation instructions will tell you. Once you have PHP complied to work with Apache there are just a couple of lines in the httpd.conf file that need to be looked at to make sure that Apache knows about PHP, and what files to send to PHP. Finally you will need to restart your Apache.
