First, can I install both v19 and v20 on the same computer?

I'm finding v20 to be very difficult. I have both v20 and v19 set to run PHP v7.3.

But PhpED is acting like it is adhering to rules for PHP v7.4 or later. My scripts don't run on v20 because it wants to keep flagging errors such as missing array elements or undefined variables, or other "small" things as errors. I never had this problem with v19. What can I do to get PhpED to just run and ignore the little things it comes across?

Thank you.

UPDATE: I guess this does the trick: $error_reporting = error_reporting(E_ALL & ~E_NOTICE & ~E_WARNING);

But I didn't have to have this with v19.

So can someone still elaborate on this issue?

PhpED does not run any scripts at all. PHP does.
Select right PHP version and it will work if it ever worked.
If you have different php 7.x version than before, either update your script accordingly or install php version you had before.