Valet switch PHP version with these commands
Install PHP 5.6 and switch Valet to PHP 5.6
valet stop
brew unlink php71
brew install php56
brew install php56-mcrypt
brew link php56
valet start
Then to switch back to PHP 7.1 it is as simple as:
valet stop
brew unlink php56
brew link php71
valet start
Then to switch to PHP 5.6:
valet stop
brew unlink php71
brew link php56
valet start
For those who're using Ubuntu Valet (unofficial), after installing certain version of php, you just need to stop the valet before switching the php version.
For me, php7.2 already pre-installed in my Ubuntu 18.04. So I installed php5.6, php5.6-fpm and php5.6-cli before I can switch between my preferred php version.