Fatal error: Uncaught Error: Call to undefined function mysql_connect()

I am trying to do a simple connection with XAMPP and MySQL server, but whenever I try to enter data or connect to the database, I get this error.

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in C:\xampp\htdocs\register.php:22
Stack trace: #0 {main} thrown in C:\xampp\htdocs\register.php on line 22

Example of line 22:

$link = mysql_connect($mysql_hostname , $mysql_username);