Downgrade php7.0 to php5.6 on GCP

Question

May anyone help me with downgrade php7.0 to php5.6 on GCP?

I tried to remove php7.0 with

sudo aptitude purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`

and install php5.6 with

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php5.6

However, I failed with the error message:

id@myproject:~$ sudo apt-get update
    W: The repository 'http://ppa.launchpad.net/ondrej/php/ubuntu eoan Release' does not have a Release file.
    N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
    N: See apt-secure(8) manpage for repository creation and user configuration details.
    W: GPG error: http://ftp.ports.debian.org/debian-ports sid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DA1B2CEA81DCBC
    61
    W: The repository 'http://ftp.ports.debian.org/debian-ports sid InRelease' is not signed.
    N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
    N: See apt-secure(8) manpage for repository creation and user configuration details.
    N: Skipping acquire of configured file 'main/binary-amd64/Packages' as repository 'http://ftp.ports.debian.org/debian-ports sid InRelease' doesn't support architecture 'amd64'
    E: Failed to fetch http://ppa.launchpad.net/ondrej/php/ubuntu/dists/eoan/main/binary-amd64/Packages  404  Not Found
    E: Some index files failed to download. They have been ignored, or old ones used instead.
id@myproject:~$ sudo apt-get install php5.6
        Reading package lists... Done
        Building dependency tree       
        Reading state information... Done
        Note, selecting 'php5.6-json' for regex 'php5.6'
        Note, selecting 'php5.6-common' for regex 'php5.6'
        You might want to run 'apt --fix-broken install' to correct these.
        The following packages have unmet dependencies:
         google-compute-engine : Depends: python3-google-compute-engine (= 20190124-3) but 2.8.16-1 is to be installed
        E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).