How to make a php the default page of a web site?

Your web host can tell you the default name set up for a file name on their server which does not have to be specified for the server to serve that file without further detail.

Normally, what's known as the home page on a website is actually a file named index.php in the root (or highest level) directory on the server (but not always)

Use index.php.

The default domain name home page is whatever your server host tells you to use.  99.9% of the time index will do the job.  If not, contact your host.

Nancy O.