Gentoo users: emerge does not compile dev-lang/php with --enable-xmlwriter by default. To enable this option, add the flag xmlwriter to your USE variable and recompile php.
The XMLWriter extension was initially a PECL extension for PHP 5. It was later added to the PHP source (bundled) as of PHP 5.1.2. This extension is enabled by default.
This extension is enabled by default. It may be disabled by using the following option at compile time: --disable-xmlwriter