Infra-Red is a package of scripts designed to set up a service oriented architecture in PHP. It includes the server with an example service, a client library for the codeigniter framework and a stand alone client library.
phleackr is a PHP binding for the Flickr API, using the XML-RPC format and thought to be compatible with PHP4 and PHP5, while avoiding the need for PEAR packages. in the future it will also support the REST format.
PHP-O-Lait is a library that, in 2 boiler-plate lines of code, allows you to call server-side PHP code from client-side JavaScript. It's so easy, in fact, that you don't need to know how it does it (hint, it uses Ajax and JSON).