This extension requires the libxml PHP extension. This means that passing in --enable-libxml is also required, although this is implicitly accomplished because libxml is enabled by default.
This extension uses Sablotron and expat, which can both be found at » http://freshmeat.net/projects/sablotron/. Binaries are provided as well as source.
Enable by using the --with-xslt option with PHP 4.