Normally any content inserted using PHP is inserted as HTML, because that’s what the browser understands. So if you use CSS to style HTML on the page, it will style all of it, irrespective of its origin.
If you post the HTML and CSS for your page (or a link to a live page) we can give you more specific advice on how to target styles to particular sections.
It’s always a good idea to post all relevant code when you ask a question; otherwise, we can only guess or give general answers. You might find it helpful to read this topic: Forum Posting Basics