Issues installing Mcrypt PHP module
The formula you are trying to install is deprecated. There was an announcement the 19 january 2018:


  By 31st March 2018 we will deprecate and archive the Homebrew/php tap. Unfortunately we have been unable to maintain an acceptable, consistent user or contributor experience and CI workload through non-core formula taps in the Homebrew organisation so we are continuing to migrate widely used formulae into Homebrew/core and encourage more niche formulae and options to be supported outside the Homebrew organisation.


Some of the PHP extensions are maintained by Kevin Abel in its Github repository but there is no mcrypt extension.

Moreover mcrypt is itself deprecated and is considered as an abandonware for nearly a decade. It is removed from PHP 7.2. Read  this stackoverflow question for alternatives: mcrypt is deprecated, what is the alternative?