This extension is a wrapper for the libevent event notification library. Formula was removed
Last versions were
php56-libevent 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-libevent: bottle revision to rebuild.
Andreas Braun Add description for libevent formula
Andy Blyler fix style issues
Andy Blyler php56-libevent: convert from sha1 to sha256

Formula code at GitHub