You’re using manually installed PHP packages that don’t stem from the official repositories. Anything could happen in that case and I suggest that you report the issue to their maintainer.
Solution 1: Switch/downgrade to in-repository packages
The immediate solution is to revert to the in-repository packages with:
sudo apt install php5=5.5.9+dfsg-1ubuntu4.20 php5-xdebug=2.2.3-2build1
If you have other packages depending on that
php5version you’ll need to install their respective in-repository versions in the same fashion.
Solution 2: Install PHP 5.6 and xdebug from an actual PPA and not manually with
How do I install different (upgrade or downgrade) PHP version in still supported Ubuntu release? has an excellent answer that lists such a PPA.
You probably want to remove the current PHP 5 installation since the PPA packages have different names to allow different independent PHP installations:
sudo apt remove php5 php5-xdebug
Add the PPA, upgrade your packages and install the new PHP and xdebug packages:
sudo add-apt-repository ppa:ondrej/php
sudo apt update
sudo apt upgrade
sudo apt install php5.6 php-xdebug
As in the previous solution you’ll need to remove packages that depend on
php5 (happens automatically with
apt remove php5) and later re-add their