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.
此扩展默认为启用,编译时可通过下列选项禁用： --disable-xmlwriter
alvaro at digdib dot com (2009-01-02 02:37:25)
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.