Die Erweiterung XSL ist seit PHP 5 in der Distribution enthalten und kann über die Option --with-xsl[=DIR] in Ihrer Build-Konfiguration aktiviert werden. Dabei steht DIR für das Installationsverzeichnis der libxslt-Bibliothek.
On Windows, if you chose "Custom" during installation; you will need to run the installation again, select "Change" in the second screen. And in the "Choose Items to Install" Screen select extensions and drop-down-select "Entire feature will be installed on local harddrive". This installs and activates xsl.
On some linux systems you might be able to simply do this:
sudo apt-get install php5-xsl
or similar.
If you want to activate this extension on Windows, uncomment ";extension=php_xsl.dll" in your php.ini file, and direct it to its respective folder. e.g. "extension=ext/php_xsl.dll"