I would always prefer a framework like Symfony over self-written code. A framework will be used by many coders, so most security issues will be found and fixed. If you want to create a product, a framework will spare you the work on the low level things like sanitization and DB wrappers. Sure, you can build them yourself, but once your “framework” is build, it must be maintained and updated to be compatible to the latest PHP version.
So, if you just write code for yourself, it is ok to use your own framework code. But if you are really serious about a product, you should use a framework!