One of the first things you learn as a PHP developer is the use of frameworks, such as the popular CakePHP and Symfony. As you use these formats throughout your career as a PHP developer, you will notice that sometimes it can be quite difficult to get what you want out of them. When you have a specific project or idea in mind that you want to create, you may not find a construction that will fit in.
As a developer, you will not have the time to sort through hundreds of different PHP structures to find the one that best fits your projects. And if you do decide to go with a framework, you could end up with resources that you do not need. In return, this will give you a bloated structure that could cause performance issues and slow down maintenance.