I am trying to build a news website.
A rich Internet application is not a particularly appropriate way to go for that. RIAs are for things like online gambling - you want a CMS, content-management system.
...I have recently been told that PHP and a MySQL database are the best for these purposes, and some have recommneded using open source such as WordPress, BUT the problem is that you are limited to what you can edit.
There are some pretty damn professional news sites based entirely on out-of-the-box WordPress modules, I don't think there is a lot in WordPress that you can't fiddle around with one way or another, and anyway you wouldn't be as limited as you would be by not knowing what you were doing. And as others have suggested, Joomla or Drupal are fine alternatives, too. All of them would give you the opportunity to pick up some programming skills while taking advantage of other people's tried and tested code.
I thus want to create my own website in PHP from scratch.
You'll have to learn PHP, then, which they say is straightforward enough if you are a programmer. In other words, if you aren't a programmer, the hard bit is not going to be learning the language but learning basic programming. It isn't comparable to HTML, it's a whole different dimension.
There is one route available to you which is a kind of compromise. Cake PHP (
CakePHP: the rapid development php framework. Home) is not a CMS but a development framework which helps you to make your own applications, including, if you put your mind to it and invest an awful lot of hours, your own CMS. I really don't recommend it, but if you can't be dissuaded, I don't know a better option.
But stick to WordPress, instead.