Hi, @edvinicombe, and welcome to the Netlify community site.
Our support team can answer questions about Netlify itself. However, answering the question of how to take an existing PHP site and convert it to a static site can be very complex. It can also be simple but, in either case, this is a question about modifying custom code/content to a static format.
I mention this because questions about custom code are outside the scope of support that our team provides (unless there is a paid support contract which specifically includes it - which is a possibility).
Someone else here in the community might be able to answer this, but our support team doesn’t have bandwidth for custom code questions. (We wish we did have bandwidth but our resources are limited so we much focus on Netlify specific issues and questions.)
If you do find a solution, we’d also love to learn about it if you are willing to follow-up here about that solution.
Finally, while I cannot recommend a specific solution for you, one common solution is to “scrape” the existing site and export it to static files. Here are a couple of links to articles which discuss those types of solutions in more detail:
If there are other questions which might be Netlify specific please let us know. (Similarly, if you are not sure if they are Netlify specific or not, it doesn’t hurt to ask. Please do ask if you aren’t sure.)