Ask HN: How do you write your own framework with user authentication?

Every time I start a project I am faced with the dilemma of "just doing it" or "doing it right" with an infinite amount of positions between. Most times I’m just looking to integrate with a sturdy user authentication system, there aren’t many other things I find myself really needing as I am developing an idea. What should I account for when building a lightweight framework for personal projects? What am I missing? Sometimes I feel frameworks are so robust I couldn’t possibly justify the need for it with a meager project, even though I would like most of these projects to flourish into an eventual revenue producing product. I would say my biggest stumbling block would be wanting to focus on building my idea in code and not focusing on the user authentication / security. I’ve often integrated custom code within a CMS, but if feels very clunky and restricting. What are some best practices for implementing your own user authentication system? I mostly code in straight PHP, but I’m not against moving to another language or a light non-obtrusive framework.