So this has happened to me a few times over the years and I always forget how to fix it. I’m scribbling it down here as much for myself as I am for you.
When you install php71-mcrypt using:
Brew will install the php71 mcryprypt extension, but when you then use php you’ll note that mcrypt doesn’t work. If you restart php and you get something like this:
Is thankfully, very very simple. Run:
Boom! Now it works.