There are plenty of these threads asking the same "what's the better framework" question with different responses, so I'll leave that to you're own discretion to research those. You mentioned though, that you were new to PHP and I thought I might have a word or two of advice. While frameworks are of definite benefit, I do find they tend to gloss over the details in order to increase productivity. They leave it up to the developer to know how and what the internals do. So, if you dive into a framework without the knowledge about how the underlying functioning works, you may end up frustrated and have to dig deeper to come to an understanding of how to debug a problem.
Frameworks seem to get a stigma for being very rigidly structured and with my experience this just isn't true. They may have conventions and requirements, but when it comes to actually making use of the code base, they don't limit my ability to solve problems, so your assertion that using a particular framework would limit your development skills set, that just doesn't hold any relevance with me. I find its quite the opposite in fact, the more I expose myself to other code, the more I adapt, find what works and continuously evolve as a programmer. I guess I should mention that I'm not fixated on one particular framework either, while I'm fond of one, it doesn't limit my curiosity about others.
I think you should have a degree of familiarity with PHP before you use a framework, primarily for the reason I mentioned before, frameworks tend to gloss over the details. I wouldn't require you to write real world applications for everything, but give yourself a practical education. I have a web server on my local network I use to test out code, write your own website, your own applications, to test, to learn. PHP is a pretty big beast nowadays so maybe come up with a curriculum of what you need to learn and what you want to learn depending on your interests.
Some topics you might want to consider (in no particular order):
Coding Standards (PEAR/Zend, MySource/Squiz)
OOP and Design Patterns in PHP
Coding Practices (Agile, Unit Testing, Test-Driven Design, version control)
Filtering & Validation (RegEx, Sanitizing Inputs)
Databases (Prepared Statements, Abstraction, Pooling, Caching, Data Integrity, Indexes)
Templates, Forms & PDFs Generation/Manipulation
Internationalization & Localization
Authentication & Authorization (Role/Group Based Permissions, Cryptography)
Mailing (SMTP, Domain Keys, PKI Signed Mail)
Security (Types of Attacks to counter)
Sessions (Security Issues)
Searching (Cached Searches, search engine, spider/crawler)
Web Services & AJAX controls (REST, JSON, Web APIs)
Caching, Performance and Scaling Issues
Debugging and Profiling
PEAR, PECL & Common PHP extensions
