Every developer thinks that their way of coding is better than every other programmer out there. They think their APIs are more elegant, their style is more succinct, and their loops are more optimized.
They are wrong.
I've had the misfortune to work with several custom-built frameworks. They were all terrible in their own way. Some had a slight excuse in that they were built in aging versions of PHP, but the otherall crappiness of the framework still stands on its own. And I can't claim that I think they frameworks were utter shit because I didn't write them, since I did write one of them.
But in writing my own framework and realizing just how crappy it ended up gave me the perspective to realize that I should use one of the big frameworks. I should use a Zend Framework, or Cake, or Symfony, or whatever. The programmers that wrote those frameworks may not individually be a much better developer than me, in aggregate they tower head and shoulders above. They get the benefit of having security experts picking at the seams of their interfaces, optimization guys fine tuning little used paths, and usability experts chiming in on workflow. They get the benefit of people trying to run the frameworks on operating systems you've never heard of, trying things with them you've never thought of, and using browsers you're too good to use.
Every developer should write their own framework, throw it away, and use one of the established ones.
It's so tempting to have the control over every aspect of the framework and the complete understanding of how each moving piece fits together. It's nice being able to custom build functionality for each small piece of your application into the framework-level code. But what always happens is that you start a new project and reuse your framework since you've put so much work into it. Then you find yourself having to code around all of the special cases that you put into the original. You're better off just using a more general framework from the start and putting your business logic where it belongs.