brew install redis
Final result
brew info redis
Gives out
redis: stable 6.0.1 (bottled), HEAD Persistent key-value database, with built-in net interface
Check your version of redis, then find a suitable version of the extension here. If unfamiliar with pecl, type pecl to see the options. Type in:
pecl install redis
Enter NO to each question asked if you're not sure.
If that succeeds check the new file it created at: /usr/local/lib/php/pecl/20180731/redis.so
The install will have added extension="redis.so" to top of your php ini. Check that by opening the file /usr/local/etc/php/7.3/php.ini. (assuming you're on 7.3 there)
brew services restart php
php -i | grep Redis