No special procedure is needed to install JSON-RPC PHP. Once the requisites are respected, that classes are normal and indipendet classes in PHP.
So, simply copy jsonRPCServer.php and/or jsonRPCClient.php in your include_path or where you want.
A path different from the include_path must be esplicitely declared when including the files.
If you have problems and you think that you are in the requisites margins, you can signal the problem to info@jsonrpcphp.org, taking care to indicate the error message received.
