I wonder if you could, for reasons, make a set-up where you upload your PHP files to an S3 bucket, and then have a API Gateway or Lambda which basically just serves the files from the bucket, and if they are PHP, executes them with a Lambda PHP runtime.

So you hit /index.php -> checks S3 bucket for index.php -> executes that with lambda and gets string of response -> returns it to original lambda -> serves up to client.