For the sake of completeness, I'll expand my comment into an answer and let you decide.

A PPA is a Personal Package Archive. Someone has taken source code from another source and packaged it so that Ubuntu users can install them easily.

Your question of "trust" comes down to what are you worried about? The fact is, any one can create an a PPA and have create a glowing homepage that looks perfect. One thing to keep in mind is that whoever makes a PPA is usually a volunteer, much like those of us who read AskUbuntu. So, there are no guarantees. In fact, the one you mentioned is described in these installation instructions, which then refer to this PPA. At this PPA, they have honestly said:

Disclaimer: there's no guarantee of timely updates in case of security 

problems or other issues. If you want to use them in a security-or-

otherwise-critical environment (say, on a production server), you do 

so at your own risk.

Does something like this pass or fail your definition of trust? It seems everyone will have different definitions of trust. In fact, what if I said "yes, you can trust that PPA!" -- but for what reason should you trust me?

My rule of thumb whether it is an Ubuntu PPA or some installation executable for Microsoft Windows is if I am concerned, I shouldn't download it. In your case, your current version of PHP should be sufficient and I wonder if you "need" features in PHP-8 or do you just want the latest version for no reason?

If you still need PHP-8, then you can just download it and install it yourself. This is the site where PHP resides. Installation instructions for this source is available here. This is the most trustworthy as you can get...