Error: php@7.3 has been disabled because it is a versioned formula

You can only install supported versions of PHP with brew. However, there is the tap shivammathur/php which can be used to install unsupported version of PHP.

1. brew tap shivammathur/php
2. brew install shivammathur/php/php@7.3
3. brew link php@7.3

The first step only needs to be done once. After adding the tap, you can install PHP version 5.6 – 8.2.