You will also need a current development version of DokuWiki. The current stable is not compatible with PHP 7.1, yet.
Is this still the case with stable 2017-02-19a?
From what I can tell I have the wiki running with php 71. Currently having a problem when I click "install" when wanting to install a plugin from within the Extension Manager.
Got the plugins working. I didn't have php71-zlib installed...