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.
The Windows version of PHP has built-in
support for this extension. You do not need to load any additional
extensions in order to use these functions.