As already noted, your definition of the scope and complexity of this project is a bit vague, but I will answer with the general remark that larger projects benefit from a larger top-down structure than smaller ones. I suspect that almost every PHP developer on the planet started hacking right in index.php, and then wrote some code for guestbook.php and so on. Then you understand how much you are repeating and start refactoring with classes and libraries.
Frames are the natural next step forward. The term covers a range of products; some of which are very closely related to a specific way of working, and some are more likely with a library with some free front-end controls.
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