By default PHP5 is not enabled on the Apache2 installation of Mac OS X Leopard. To enable it edit /private/etc/apache2/httpd.conf and uncomment the following line:
LoadModule php5_module libexec/apache2/libphp5.so
I needed to enable SOAP on my PHP installation but attempts at using the instructions from Pagebakers did not work out too well. I managed to compile the /apple/osx/leopard/php/soap.so extension for PHP 5.2.8 but adding it to php.ini only resulted in it showing form the CLI when using php -m.
You'll find a pre-compiled version on the site which I've mirrored here as /apple/osx/leopard/php/soap.so for myself. This module was copied into /usr/lib/php/extensions/no-debug-non-zts-20060613 and extension_dir set to this in /private/etc/php.ini as well. From there just add a 'extension=/apple/osx/leopard/php/soap.so' and you're sorted.