The simplest form of REST with PHP comes with XML + CURL. PHP can use the features available within the language to send and receive data, preferably in XML format in case of Web services, over HTTP. One of the most popular techniques is to use the CURL API and GET or POST contents to a URL. This naturally maps to the REST principles. This model is for client side, for consuming services.
For server side, going by the REST principles, one could argue, any hosted PHP script could be considered a REST service. Again, for the sake of Web services, to enable machine to machine interactions, rather than machine to human, XML would be the preferred form of input and output in case of a service. Again, nothing fancy is required, other than the current set of PHP API and extensions.
In short, REST with PHP is pretty simple.