The PHP extension for the gRPC library Formula was removed
Last versions were
php56-grpc requires the following formulae to be installed:
autoconf 2.69 Automatic configure script builder

Recent formula history

ilovezfs php56: migrate to homebrew/core (#4835)
Stanley Cheung php-grpc 1.6.0
Stanley Cheung grpc: update to 1.3.2
Stanley Cheung php-grpc 1.2.0
Stanley Cheung gRPC: Update to version 1.1.0 and add formula for PHP 7.0 and 7.1

Formula code at GitHub