In my experience, the most important thing for starting a solo project is an idea. Environment, docker, front or backend priority, plugins and all that come second to knowing what you want to do.
Next, to me, would be the environment setup. That will often have the largest production impact on how the project performs. Assuming you know the language etc already - You said PHP, which is a perfectly fine language for web but the language decision is usually made at the project decision time as different languages have different specialties - then the next biggest decision is how it is deployed as deployment config can change a lot.
Which brings us to docker. Docker is good to use in most cases. Its better if you need multiple different environment configs. Say, for example, a library relies on php 5.x but you want to use 7.x for the rest of it for performance reasons. Docker lets you use both by deploying two containers on different versions for different parts.
Backend vs frontend: Your choice, Really, and it also depends on the project. some are more inherently frontend or backend oriented, but a good framework for either end can smooth out a lot of either sides problems. I prefer backend-first because understanding the data flow of the app is more important to me than how it looks. nothing a bit of css and js can't fix, usually.
plugins last. anything that doesn't actually affect deploy code is the lowest priority, but still a priority if it means improving how quickly i can write the code. Performance and architecture are more important. To me, anyway.