The PHP client is provided by Webthumb.
The source is managed at github, information on how to clone the repo and tarball downloads of any version is provided there.
In general its just:
git clone git://github.com/jeichorn/Bluga.net-Webthumb-API-for-PHP.git
The last release before the move to github is: svn-rev 62
The examples directory shows how to use the client, start there after installing the code.
Note that as of 2008-12-10 this wrapper is using api version 2, this is an incremental API upgrade that returns errors as XML. If you want to see similar results in your testing just add a version tag with a value of 2 to your request. Using the debug mode of the apiwrapper will also show you all the XML that is sent to the server. If you have any problems please include debug output in your emails.