I ran composer install today, and was rather surprised to find that the intl module wasn’t installed?! Then I remembered I had installed package updates etc, and it turned out I had BOTH PHP 7.3 AND PHP 7.4 installed!
All my sites were developed on 7.3, so I had to switch back.
update-alternatives --list php
Typing that command in will output something like this:
/usr/bin/php.default
/usr/bin/php7.3
/usr/bin/php7.4
Now you can see the available versions. To switch versions, I simply typed
sudo update-alternatives --set php /usr/bin/php7.3
DelBoys Place 
I’ll definitely visit this blog again, thank you 🙂