I saw the below post on macosx.com by Casey and it worked for me. After running the package you need to only do a couple of things.
1. Add the following lines to /Library/WebServer/Configuration/apache.conf
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LoadModule php4_module /System/Library/Apache/Modules/libphp4.so
AddModule mod_php4.c
2. Un-comment the following lines from the same file
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AddType application/x-httpd-php .php
AddType application/x-httpd-php-source .phps
3. Restart apache.
An excerpt of the origional post is as follows
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I was amazed to have installed PHP4 last night via an OS X package... no apache config or anything... it was so easy I almost feel violated... this PHP4 package is available from Tenon (http://www.tenon.com/) It is distributed along with their iTools beta.