I assume an expert will get you squared away soon, but I thought it would be fun to help with a little more understanding than you seem to have now.
I don't know what is xamp, but if you plan to write PHP on your own computer and also test what it does on your own computer, then you need to install PHP in a way that it works together with a web server right there on your computer. Once you have that installed and running properly, then you would save the web pages you write with notepad (or other software) in a folder wherever you want. But you would want to pick a folder which you have told your webserver to serve as your "local" website. Then you just open that page in your web browser just as if that was a website on the internet, only it actually lives on your own machine. You put the PHP code right into your webpage and simply put the address of that page into your web browser. The way it is all set up on my mac, I can access my local PHP driven "website" by either of 2 addresses:
(the IP address of this (my) computer)
this latter address works because I told my webserver Apache (Apache comes already installed on new macs nowadays) to open the default page (index.html) whenever I type in "http://mylocaltests/".
The former address works even without any setup. All computers have an IP address (those numbers "192.168.1.100").
Step by step you will understand the needed things and get it running.