interface to the libeio library Formula was removed
Last versions were
php54-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

Mike McQuaid Deprecate tap.
Andy Blyler Fix php5*-eio on OS X 10.11
Andy Blyler Update URLs to use HTTPS
Jack Nagel Move abstract classes out of Formula directory
Andy Blyler add pecl eio to php5[3-6]

Formula code at GitHub