[rudra]

first things first..
"debian 9 release is " testing/unstable" Sid"
NOT TRUE!!!!!!

release code name / alias can be - jessie, stretch, buster, sid etc. these are fixed.

release class can be - oldstable, stable, testing, unstable. these indicate code names...true. but they keep changing with time. Right now,
Deb 8 jessie is oldstable
Deb 9 stretch is stable
Deb 10 buster is testing
beyond Deb 12 lies unstable or sid.

If you don't want to upgrade to buster when it launches (= becomes stable) then use code name 'stretch' after source url. You will stay with debian 9 stretch all the way to its end of life.
Example,
deb http://deb.debian.org/debian/ stretch main
If you use 'stable' in place of 'stretch' then it will (or try to) upgrade the software to its stable version when you run 'apt update'. that means when Deb 10 buster gets stable (and that means Deb 9 stretch becomes oldstable) your software will upgrade to its Deb 10 version (or throw error if dependency issues arise).

If you want to use newer versions available in 'testing' then use it there. Your system will keep updating accordingly. But meeting the dependencies for them and keeping the system in working order will be no small task.

NOW CHECK YOUR OLD POSTS HERE FOR THE SOURCE URLS. YOU HAVE SOME MAJOR MIX UP.

I even see this
http://archive.ubuntu.com/ubuntu trusty InRelease

don't think trusty is a code name associated with Debian. hehe
It is Ubuntu. The URL too.

straighten it up and clean the package cache. Hopefully that will solve the problem unless you have a piece of software that is keeping an old version of something that is breaking things.....or something else.

Also WTBD is suri.org ?
is it a main repository for your php for debian?

Personally, I always use main USA or UK repository. Always.

Source.
https://en.m.wikipedia.org/wiki/Debian_version_history


EDIT.

SO clearly your solution is use 'stretch' and not friggin 'testing'. that is why it says thise installations require newer packages but your installable is older. mismatch.

Dean...

CentOS is good. But I love Ubuntu and never had a problem I didn't manage to google and solve.