Hi,
If you choose using a framework, Not depending on which one you'll chose, the first thing you have to know is that :
- it'll take some time for you to know it
- you'll do crapping things, during that time ^^
- so, take into account the fact it'll take some time before you are fully operational :-)
I think those points are the most under-estimated points : using a framework takes not much time... Using it well and to the full extend of its abilities takes... Well, a couple of months, maybe... Which means, when you are at the end of your project you'll say "I should rewrite that with all the stuff I learned while re-writing it the first time" :-D
What it means is : learn what the framework can do, learn how to use it, and use it for a while on small applications, before starting rewriting your big one !
(Disclaimer : this previous paragraph was a re-post of a part of an answer I did to this question)
On the other side, if you don't use a framework :
- it will still take time to know PHP well enough to develop your application
- you will still do crapy things during that time ^^
- and you should still spend some time on "little projects/applications" before starting programming the big one, to get familiar with the language.
So, actually, using a framework or not, the same stuff is still true : using a framework only gets you some boundaries, help, and pointers -- and a lot of already existing great code !
Finally, about the question of "using a framework" vs "not using a framework"... Well, it's 2009, and considering the amount of things/code/best-practices/guides/whatever a framework brings, I'd definitly go with a framework : I work in a company that develops websites, and we don't ever ask ourselves the question "should we go with a framework" -- we still go with the question "which framework is the best for this project", though.
Plus, if you are willing to work (ie, professionnally speaking) in PHP development, having used a "big" framework would be a plus on your resume (well, I don't know about the country where you're from, but, in France, it is definitly a plus)
(Added as an edit, to be more clear about what I meant)
Still, while I'd say "use a framework" for your application, that definitly doesn't prevent you from first spending/investing some time learning at least the basics of PHP, it's syntax, and some of it's gotchas -- but that could be on smaller "example applications", I believe.
And, if you are curious about "which framework should I choose"... Well, that's quite a debate ^^
About that, instead of debating all over again, here are a few pointers to other questions/answers :
(Well, how strange is that : some of those might also help you answer your own question ;-) )