I don't judge the language for its age or quality, but rather its relevance or popularity. I asked around and only one of the software engineers I know has even touched PHP in the last 10 years, and online polling agrees that it is not very commonly used. I know there are still PHP developers out there.
The reason I am taking about this is that there are going to be very few people able to work on this in a meaningful way. Sure, I could catch up on 15 years of changes and relearn the language. Unfortunately that means that I and many others will be approaching this from that perspective (i.e., the perspective of a relative novice) rather than as subject matter experts. Progress will be slow and many are not going to want to invest the time. The best bet for making an impact is to find passionate PHP developers that are also passionate about social media and decentralization. I imagine that venn diagram has a very small overlapping area, and conversations and DMs I've had with others on these platforms seem to point to the same conclusion.
Scale is also a concern (and already a real problem), but this has more to do with the architecture of the application than the language itself.