Programmer Humor

I mean, I'm a bit out of my water there, both in terms of the featureset of PHP and what's actually needed for a kernel, but I'm still gonna go with no.

For one, PHP uses reference counting + garbage collection for memory management. That's normally done by the language runtime, which you won't have when running baremetal.

Maybe you could implement a kernel, which does as few allocations as possible (generally a good idea for a kernel, but no idea, if it's possible with PHP), and then basically just let it memory leak until everything crashes.
Then again, the kernel is responsible for making processes crash when they have a memory leak. Presumably, our PHP kernel would just start overwriting data from running processes and eventually overwrite itself in memory(?). Either way, it would be horrendous.

Maybe you could also try to implement some basic reference counting into your own PHP code, so that your own code keeps track of how often you've used an object in your own code. Certainly doesn't sound like fun, though.

Well, and secondly, I imagine, you'd also still need an extension of the language, to be able to address actual memory locations and do various operations with them.

I know from Rust, that they've got specific functions in the stdlib for that, see for example: https://doc.rust-lang.org/stable/std/ptr/index.html#functions
Presumably, PHP does not have such functions, because its users aren't normally concerned with that.