These functions are enabled by default, using the bundled expat library. You can disable XML support with --disable-xml. If you compile PHP as a module for Apache 1.3.9 or later, PHP will automatically use the bundled expat library from Apache. In order you don't want to use the bundled expat library configure PHP --with-expat-dir=DIR, where DIR should point to the base installation directory of expat.
La versión para Windows de PHP tiene soporte nativo para esta extensión. No se necesita cargar ninguna extensión adicional para usar estas funciones.