If you are using PHP 7 within your XAMPP then it wont work because that code is too old. mysql_select_db was removed in PHP 7.
You will need to install PHP 5 to use that software.
Edit: Pratik Solanki is correct also. The database is being connected using mysqli so depending on the other code in the system you can either change the database connect statement to mysql_connect and use PHP 5 (old mysql connector not recommended) or change all the database statements to use mysqli instead in which case no PHP changes are needed (recommended)