Legacy MongoDB database driver. Formula was removed
Last versions were
php56-mongo requires the following formula to be installed:
autoconf 2.69 Automatic configure script builder

Recent formula history

ilovezfs php56: migrate to homebrew/core (#4835)
Andreas Braun Mongo 1.6.14 for PHP 5.6
Andreas Braun Update mongo formula for PHP 5.6
Andreas Braun Add formula for new mongodb driver
Andreas Braun Update mongo driver to 1.6.12

Formula code at GitHub