Presumably you know that you need to create your project page in the folder <root>\xampp\htdocs, where <root> is wherever you have installed your xampp (e.g. c:\my_web_stuff\xampp\htdocs) ?
Put your index.php file into the above location and you should be ready to go. If it doesn't work I'd recommend putting a simple plain HTML file there something like this and let's assume that you save the following to the file "c:\my_web_stuff\xampp\htdocs\hello.html"
<head>
<title>My XAMPP is working</title>
</head>
<body>
<p>Hello world!</p>
</body>
</html>
You should be able to open the above in your browser at http://localhost/hello.html or at 127.0.0.0/hello.html. If that doesn't work I'd question whether your XAMPP is actually working. Go into services (I'm assuming windows) and check the the Apache service is up and running and that you aren't running into a need to enter your sign-in credentials for the service, which is sometimes the case.
The
Apache error log should be checked if you're still having problems.