Apple has included PHP 4.06 in OS X 10.1. Stepwise has the instructions necessary to make it functional - just uncomment the following four lines in /etc/httpd/httpd.conf:
LoadModule php4_module libexec/httpd/libphp4.soStepwise has a nice page of info on changes in 10.1 that covers this and other aspects of the upgrade; give it a visit to see some things to keep in mind as you upgrade.
AddModule mod_php4.c
AddType application/x-httpd-php .php
AddType application/x-httpd-php-source .phps
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010928171047621