The XMLReader extension is available in PECL as of PHP 5.0.0 and is included and enabled as of PHP 5.1.0 by default. It can be enabled by adding the argument --enable-xmlreader (or --with-xmlreader before 5.1.0) to the configure line.
This extension is enabled by default. It may be disabled by using the following option at compile time: --disable-xmlreader