Maintaining a core PHP project involves keeping the codebase organized and up to date. Here are some steps you can follow to ensure that your project is well-maintained:

1. Use version control: Use a version control system like Git to track changes to your codebase. This will allow you to revert to previous versions of your code if necessary, and make it easier to collaborate with others on the project.
2. Follow coding standards: Adhere to established coding standards to make your code easier to read and maintain. This includes using descriptive variable names, commenting your code, and using proper indentation.
3. Keep dependencies up to date: Make sure to regularly update any dependencies (libraries or frameworks) that your project relies on. This will ensure that your project is using the most stable and secure versions of those dependencies.
4. Write tests: Write automated tests to ensure that your code is working as expected and to catch any regressions. This will make it easier to make changes to your codebase without breaking existing functionality.
5. Monitor performance: Use tools like Xdebug or New Relic to monitor the performance of your code and identify any bottlenecks or areas that can be optimized.

By following these best practices, you can keep your PHP project well-maintained and running smoothly.