What is better to keep, keep the core of the system in each project or take it to a higher level (for general use)?

Question

PHP

There is a server on which there is a sufficient number of projects, all projects have a common core that 90% does not change.
Accordingly, what is better:
1. move the kernel several levels higher in the file system and leave 10% of the code in the projects themselves. Accordingly, it's easy to connect a common core
2. Do not take it out to the general one, but use the same code in each project.

Answer 1

Take the core to the composer package and include it on each project where it is required.