Because php 5.6 is EOL from 01.01.2019, it was removed from homebrew.
How can I install it with homebrew even if it is not in core?
brew tap exolnet/homebrew-deprecated
brew install php@5.6
source: https://joostvanveen.com/a-29/install-and-configure-multiple-php-versions-on-mojave-with-homebrew