This extension is enabled by default. It may be disabled by using the following option at compile time: --disable-xml
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.