I just know pure PHP, never worked with a framework before, but my boss wants me to create our next project, which will be a lot bigger than everything we did before (means bigger than a page that needs only 5 scripts to work, more like 100 or something like that).
But I'm not sure if I can realise it with pure PHP, now I heard that CakePHP could be helpful for that (structure etc.). Should I learn this or just use my pure PHP way?
If you know PHP, you'll learn more about how to implement common design patters if you start reading other peoples code. The more you read, the more you'll get perspective on what good code looks like, and what bad code looks like.
It is tempting to "code-your-own" because you will understand it more fully, but remember, you're not going to be the only one working on this, and one day, you might actually want to work on something else, and having a system which is widely implemented and understood can make it easier to move on.
As far as whether cakePHP a good choice for your particular project, it's hard to say without knowing more than it's going to be "bigger".
I do not say CakePHP. But any framework will do. The major benifit will be you will get a default folder structure, skin, language framework (oscommerce etc. do). You will be customizing this framework. Most features will/may there be by default, like email sending, page creation, Menu generation etc.
You did not say what kind of application you plan (may be because of NDA). More details, more accurate answers.
Everything depends on the project and the project stake holder. If there is going to be a lot of modification, either during development or after the project goes live, a framework can be pretty limiting to someone without a strong grasp of the theories behind the framework (not a judgement on the programmer, just a statement.)
As noted, your definition of the scope and complexity of this project is a little vague, but I'll respond with the general observation that larger projects benefit from more "top-down" structure than smaller ones. I suspect that pretty much every PHP developer on the planet started by hacking straight into index.php, then wrote some code for guestbook.php, and so on. Then you realise how much you're repeating yourself and start refactoring to classes and libraries.
Frameworks are the natural next step up from that. The term covers quite a range of products; some that tie you very closely to a specific way of working, and some that are more a library with some loose front control.
I'd advise you to choose a loose MVC framework, which gives you a good structure to work within, but doesn't overly constrain you, and should allow you to use existing libraries. I've not used CakePHP - my experience is with Zend Framework, which I like a lot (not that it's flawless). However, I have worked with another developer to compare the functionality of Cake and ZF, and from what I've seen Cake has many of the strong points that ZF displays. In fact, in many places it almost seems you could convert code from one to another by changing a few classnames.
I suspect Cake's not a bad choice at all, but I can't recommend it as I don't know enough about it. ZF I do know, pretty well now, so I can recommend it - and the docs are now pretty good.
Before you dive into either Cake or ZF, you'll need some understanding of the MVC design pattern. Jason Sweat's book is a good, if slightly dated introduction, and the ZF manual is also pretty strong.
By the way, it's not a choice between "Cake and Pure PHP". Cake (and ZF) are both "Pure PHP". The difference is between "PHP I wrote", and "PHP someone else wrote" (so I didn't have to). From this, the important bit is that you trust the quality of that "someone else's code", which in this case you'll have to do by recommendation and reputation.
But don't just go asking "What's the best PHP framework?" - that's like asking for the best text editor ;) And I'm sure this response (or question) will get tagged 'subjective' in mere moments.
I agree that it shouldn't be a decision whether to use CakePHP, but whether to use a framework at all. There are a multitude of PHP frameworks out there (Cake, Symfony, Codeigniter, Zend, etc.) and each has it's advantages and disadvantages.
If it was me, I'd use a framework just to strictly enforce some MVC rules... with a large project, keeping things structured helps down the road when someone else inherits the system.
The key here, is don't get lulled in by the "code generation" aspects. If you don't know the framework (or don't know the language as well as you could) down the road, the code generation could be more of a headache than it's worth.
Using a PHP framework like cakephp will reduce the amount of code you need to write. In cakephp there are many things that will save time like using elements to display snippets of code that you use often. If the project is really big, it will help tremendously because your code will be shorter and much more clean and organized. It will be much more easy to maintain as well.
I've been using Cake for a couple of years. I'd say if you haven't used any MVC framework before, you should definitely learn one to expand your horizon.
It's not really about using the framework for your convenience. It's about seeing how things are done in the framework-land. You'd really gain some insights that you can carry over even if you later decide not to use a framework.
Your description of the project didn't sound too huge. However, learning an MVC framework is some serious learning curve there. Basically you'll feel like you're learning a whole new programming language. So for your situation if you really do decide to try a framework, you might want to factor that in your schedule.
To get you started, I personally think CakePHP has a more elegant solution and has tighter design philosophies. CodeIgniter, on the other hand, seems more natural to "pure PHP-ist" because it's less abstract so it might be easier to pick up.
To answer the titled question though, I'd think any huge project should roll their own from the start instead of using a framework. The whole point of using a framework is because you want to leverage some of the stuffs they have built in for you. But for really huge projects, many of the parts should be customized anyway, so you might found yourself restricted by the framework more than leveraging it.
But then again, I really don't think your project (by your description) is anything near huge. As a ballpark estimate I'd say any project under 50k - 100k hits a day can safely benefit from using a framework.