At its most basic, index. php is the first file that will show up when a browser goes to the site or a directory. Beyond that, it can be used for whatever the site/project dictates. For more complicated sites or frameworks, it acts as the "boot" script.

The tracking code should be added to every page you want to track. In a PHP website, this is often done by inserting the code into a header file that's included in every page. Make sure teplace G-XXXXXXXXXX with your actual Measurement ID!

If your Google Analytics isn't showing data, the problem might not lie with GA4 or script issues at all. It may be an issue with your website. The issue here isn't that GA4 is malfunctioning; it's that your website isn't optimised for SEO properly and therefore receives little or no traffic.

Yes! Provided these technologies serve pages that are visible in a browser, without special plugins installed or enabled, Google will generally be able to crawl, index, and rank them without problems.