I wouldn't stay on PHP 5.6 seeing how it stopped getting security patches in 2018.
The ManageSettings.php issue I've seen before, the line has $nul, $nul, $nul in it, you can safely change this to $nul, $nul2, $nul3. Try changing that before going further.
There are no relevant extensions in 5.6 that wouldn't be available in 7.x (except mysql, which is replaced out for mysqli in 2.0.14+)