You need to install a newer version of PHP and use it for your phpmyadmin. You can still keep your ancient version installed for your legacy application.
It already has the newer version installed. I want to know how to install an older version that will work with PHP 5.6 as the app that I wish to install only runs on 5.6 and below.
I just did a fresh install of Hestia CP yesterday on a new VPS. I already have it running on another VPS which I installed around 18 months ago and at that time the version of phpmyadmin that came with Hestia worked ok with PHP 5.6 but now apparently it does not.
Why are you expecting phpmyadmin to work with an ancient version of PHP? Furthermore, why do you even want to? Just install multi-php and use a supported modern verson with phpmyadmin and the obsolete version with your legacy application.