I set up a test box and updated to PHP7.3 and it works. No need to fiddle with documentElement and links work. So the details here https://www.tecmint.com/install-differe ... in-ubuntu/
about installing different version of PHP can be used to fix.
NOTE: If the "sudo apt install python-software-properties" says "not found" you can ignore it
I'm working on testing that theory on the server.
That works, though for some strange reason I need to leave the extensions commented in the 7.3 php.ini or I get different errors.
To give the brief version:
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt install php php7.3 php7.3-xml php7.3-json php7.3-mysql
# at first that didn't work then i ran a "sudo apt-get upgrade" (be careful with that), then I went to software updater and it pulled down 7.3
# that seems weird to me, so YMMV
# set the default version for the cli
sudo update-alternatives --set php /usr/bin/php7.3
# set the default version for apache
sudo a2dismod php7.2
sudo a2enmod php7.3
sudo systemctl restart apache2
Like I said, it bugged if I updated the 7.3 php.ini to uncomment extensions, but worked if I did not. I'm not sure why that is.