The library has been designed with goals of scalability and backward compatibility. As such, it supports a wide range of PHP installs. Note that not all features of the lib are available in every configuration.
The minimum supported PHP version is 5.1.
If you wish to use SSL or HTTP 1.1 to communicate with remote servers, you need the "curl" extension compiled into your PHP installation.
The "xmlrpc" native extension is not required to be compiled into your PHP installation, but if it is, there will be no interference with the operation of this library.