I'd strongly recommend getting started with a framework, once you're comfortable with the basics. This means that you should probably work a few examples from a book first, and get a good grounding in object oriented practices and relational databases. Once you've hacked about a bit, move on to frameworks as soon as possible.
All frameworks I can think of in the PHP world are strongly object oriented, so provided you're comfortable with that, you should be in a good place to make a start. The advantage with using a framework is that you can learn best practices, which is especially important in the PHP ecosystem - as a PHP programmer myself I can tell you that there are a number of poor practices that are passed around on blogs etc, and it will pay to learn from the correct sources.
I'd suggest learning from a micro framework like Silex to begin with - this is placed somewhere between the middle of straight PHP and full blown frameworks like Symfony, and you'll be introduced to the MVC paradigm as well as great other constructs such as the use of ORMs and lambda functions.
And to answer the second part of your question, the main benefits are that:
- the code is highly tested,
- it introduces best practices,
- most frameworks encourage test driven development, which helps you output reliable code, 4. many frameworks emphasise separation of concerns, making applications scalable and easier to debug,
- most frameworks have drop in plugins or libraries for doing all sorts of wonderful things, which can really speed up development, and
- many frameworks will generate code for you - you give them a database definition and they will generate all access code (CRUD)