hella legacy js and php is the opposite of intertwined by it's very nature. every page is isolated. one php file per page. probably some inline js script tags.

hella legacy js and php is the opposite of intertwined by it's very nature. every page is isolated. one php file per page. probably some inline js script tags. easy refactor. and if they're actually using frameworks and libraries.. well that's just being a good programmer. if you don't know how it works, leave it alone and go rewrite a piece that doesn't require rewriting something core