mcrypt for PHP 7.4 on Catalina
Don't know if this is the right way but I got it to work:
https://stackoverflow.com/questions/54028143/mcrypt-php-extension-required-on-mac-os-x-mojave

The answer on there was to pretty much remove any link for mcrypt and then re-link it. I had issues with the autoconf which required some file permissions tweaking (sudo wasn't working). That answer was here. https://stackoverflow.com/questions/16844826/error-when-trying-to-brew-link-autoconf

This was such a nightmare but that worked for me.