Ok so the repo to use is: deb.sury.org. There is a readme in the base of the repo which will assist you to set it up. Although, personally I prefer to do it slightly differently. This is what I would recommend on a v16.x server (all run as root):

wget -O /usr/share/keyrings/php.gpg https://packages.sury.org/php/apt.gpg
cat > /etc/apt/sources.list.d/php.list <<EOF
# DEB.SURY.ORG repo for php
deb [signed-by=/usr/share/keyrings/php.gpg] https://packages.sury.org/php/ buster main
EOF

Then you should be able to run an "apt update" and "apt install ..." your desired packages. It's probably a wise idea to take stock of which php packages are installed, before you make any changes. This will write them to a file:

 apt list --installed | grep -i php > /root/default_php.txt

So now you can check that file (i.e. 'cat /root/default_php.txt') to work out which packages you will need in the alternate PHP version. Note that the list will include any packages that include php in the name (so may include non PHP packages that include php in the name - it should be zero, but just in case). You should be good to just install them via apt.

It's perhaps obvious to some, but I think it's worth noting that you'll need to ensure that the relevant php version of packages are installed. E.g. If you are upgrading to PHP7.4, then having "php-mysql" won't do what you expect, you'll actually need to install "php7.4-mysql".

For use with Apache, the all important thing is not just ensuring that you have installed the newer Apache mod_php (libapache2-mod-phpX.Y - where X.Y is the desired PHP version, e.g. libapache2-mod-php7.4) you'll also need to enable it. Assuming PHP7.4, do that like this:

a2dismod php7.3
a2enmod php7.4

Then restart Apache:

systemctl restart apache2

If you use PHP via commandline, then you'll also no doubt want to update that too:

update-alternatives --config php

That should list any available PHP versions and allow you to select the one that is used when you run 'php' from the commandline.

Note that if you have made tweaks to the PHP config, you'll need to reapply them to the new PHP config file (IIRC should be /etc/php/X.Y/apache2/php.ini or /etc/php/X.Y/cli/php.ini for Apache and CLI config respectively, where X.Y is the PHP version). The final thing that I can think of is updating the config file paths in Webmin (if you use it). I suggest that you replace the default paths with the new PHP config file paths. I don't recall exactly how to do that, but IIRC it's pretty straight forward.

Don't forget that once you do that, you'll no longer get the auto security updates for PHP. You'll continue to get other Debian OS sec updates, but will need to manually install any newer PHP updates. Note too that it's likely that most updates won't be security related, but there won't generally be anyway to tell (unless you read the changelog). The main reason why we recommend not auto installing anything other than security updates is because new releases can also bring new bugs (carefully backported fixes contain significantly less changes, so also less room for new bugs). So be sure to always have a current, tested backup (I recommend a snapshot if/when possible) ready just in case something goes wrong. I wouldn't anticipate any issues, but you can guarantee that if you skip taking a backup one day, that will be the day there a serious bug in a new PHP release!

Hopefully that gets you going. If you get stuck and/or have concerns or questions, please post back. Good luck.