XAMPP is just a packaged version of PHP, Apache and so on which is easy to install. The PHP version in XAMPP may be different to the PHP version installed on a web server as well as the PHP configuration and available modules. You can edit php.ini in XAMPP to control some information but version wise unless you have your own server where you can install the same version of PHP you'll have to code with a thought to PHP version differences.
You may call xampp as a subset of apache but not vice versa. Various server configuration variable can set through editing of php.ini file.