I would like to host a website using a static site provider (Cloudflare Pages, Netlify, etc), is it possible to use PHP on these? I have considered using a cdn akin to how Bootstrap is distributed that could be put directly in to the codebase, but I cannot find one that is available.
If this is not a possibility, then why can I not run PHP code using a CDN?
You have a fundamental misunderstanding somewhere.
A CDN is a service that provides geographically localised, static files that browsers can download quickly.
PHP, in the context of the WWW, is a server-side programming language. It is never delivered to browsers. It can’t be included by a line of HTML.
Consequently, the idea of a CDN for PHP doesn’t make any sense.