Can't upgrade debian stretch php to 7.x [closed]

Solution 1:

Nothing is broken. Debian 9 is simply not supported anymore, and has never included PHP7.3.

Debian 9 has been superseded by Debian 10 ("buster"). Security updates have been discontinued as of July 6th, 2020.

From Debian's page on Stretch. In short; upgrade to Debian 10. Debian 9 is not supported any more, and running a web server on a unsupported OS is crazy.