interface to the libeio library Formula was removed
Last versions were
php56-eio requires the following formulae to be installed:
autoconf 2.69 Automatic configure script builder
libevent 2.1.8 Asynchronous event library

Recent formula history

ilovezfs php56: migrate to homebrew/core (#4835)
ilovezfs php56-eio: bottle revision to rebuild.
Andy Blyler Fix php5*-eio on OS X 10.11
Andy Blyler php56-eio: convert from sha1 to sha256
Andy Blyler Update URLs to use HTTPS

Formula code at GitHub