I agree with the sentiment that it’s your course to set and the major consideration is your own personal sanity. I will say, however, that if your project is used by many projects then one consideration should be their adoption strategy. Maybe it makes sense to find some usage stats, or maybe a survey, and figure out what versions of PHP your users are currently using and expect to use throughout the course of your roadmap. You may find you need to support lower versions, or that there’s enough appetite to keep things current that you can work towards bleeding edge support.
More generally, most libraries will work to not introduce breaking API changes and will just peg their latest versions to newer technologies, with the expectations that people can use an older version that satisfies the php requirements.