I see a few issues or potential issues:
1. Using Windows. If your application will eventually be deployed on Linux, then you really need to develop it on Linux due to various differences in the way PHP works on the platforms. It's also commonly suggested that PHP is slower on Windows. There are also issues with some important PHP extensions on Windows. If you don't have a native Linux machine, then use https://www.virtualbox.org/ to create a VM.
2. Using a tnsnames.ora file. Why not use the Easy Connect connection string format "hostname/servicename" that doesn't require any local configuration file? This removes one point of failure because you don't have issues making sure Oracle libraries can locate and read the tnames.ora file. (For the syntax, see PHP: oci_connect - Manual or my book The Underground PHP and Oracle Manual)
3. Nothing in your question involves PHP or Netbeans. Since tnsping fails, you have a basic network or Oracle client installation issue. This needs to be resolved first, before looking at PHP (though make sure to use the correct 32 or 64 bit as required by Xampp). See the next point for what I suspect is the problem.
4. Your list mentions Oracle 11.2 and Instant Client as separate items. If you already have a DB on the machine, then you don't need Instant Client (unless there are 32 vs 64 bit issues). Having multiple sets of Oracle libraries on Windows is a common source of user errors due to library clashes. Clean up the machine and check connection using SQL*Plus from the DB home.
5. In the last few months Windows PECL extensions have become more easily accessible from http://pecl.php.net extension pages, so the need for bundles like Xampp has decreased. It might be easier to install Apache and PHP separately. See http://www.oracle.com/technetwork/articles/dsl/inst-php-apache-windows-098333.html