On his site Michael Dyrynda shows you how to switch PHP versions in Laravel Valet away from the current default of PHP 7.1 (should the need arise).
At the time of writing, Laravel Valet ships with PHP 7.1 but if you're like me, you have some legacy projects around the place that haven't quite lifted their dependencies to PHP 7 just yet.
A lot of folks might have previously used a VirtualBox Virtual Machine, or more recently considered Docker but a lot of the time and especially when dealing with simpler situations, Valet may be all that you need.
Luckily, the combination of Valet and brew on macOS makes switching PHP versions really easy.
With the help of the Homebrew tool it's as easy as telling it to install the PHP version you require and create the link to this needed version. He's included two screencasts in the post to show the process and commands you'll need to accomplish it.