Building your own framework can be useful for the following reasons:
However, on the PHP market there are many very high quality open source frameworks that many projects and organizations use to build software faster and better. Picking an established open source framework has its benefits:
For creating a framework you will have to know OOP, design patterns, ORM, authentication, MVC (model view controller) and other advanced concepts.
Here are listed some useful resources and tutorials that can also help you create your own modern and quality PHP framework.