This extension has been moved to the » PECL repository and is no longer bundled with PHP as of PHP 7.4.0
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » https://pecl.php.net/package/wddx.
After installing the required expat library, compile PHP with --enable-wddx, and use --with-libexpat-dir for 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.