There are a few different options for installing PHP with Apache on Mac OS X, including building from source. However, the simplest method is to install PHP from a precompiled binary package, namely the ones distributed by a developer named Marc Liyanage.
By the Way
If you would like to build PHP from the source and are comfortable with the Mac OS X operating system and using the terminal, feel free to follow the instructions in the PHP manual, at http://www.php.net/manual/en/install.macosx.php.
To obtain the PHP installation package, go to http://www.entropy.ch/software/macosx/php/ and look for the section titled "Installation Instructions." Once there, follow the link to download PHP 5 on Mac OS X 10.3.
When the download is complete, mount the disc image and double-click on the *.pkg file. Follow the installation steps below to complete the process.
The PHP installation program has made all the necessary changes to your Apache configuration, so there's nothing left for you to do except skip ahead to the "Testing Your Installation" section.