Instead of asking 'how to use PHP/MySQL to let users affect webpages' I'll ask this, because I learn better from projects and examples.
So how would I incorporate a VERY basic comment feature using PHP and MySQL?
It's a hard question to answer without more information. There are a number of things you should consider when looking at implementing commenting on an existing website.
How will you address the issue of spam? It doesn't matter how remote your website is, spammers WILL find it and they'll filled it up in no time. You may want to look into something like reCAPTCHA (http://recaptcha.net/).
The structure of the website may also influence how you implement your comments. Are the comments for the overall site, a particular product or page, or even another comment? You'll need to know the relationship between the content and the comment so you can properly define the relationship in the database. To put it another way, you know you want an email address, the comment, and whether it is approved or not, but now we need a way to identify what, if anything, the comment is linked to.
If your site is already established and built on a PHP framework (CakePHP for instance) you'll need to address how to integrate your code properly with what is already in place.
Lastly, there are a number of resources and tutorials on the web for PHP. If you do a quick google search for something along the lines of "PHP blog tutorial" I'm sure you'll find hundreds and the majority will show you step by step how to implement comments.