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

You probably have a php7 in XAMPP Please use PDO.
mysql_* functions have been removed in PHP 7
You can use

mysqli_connect($mysql_hostname , $mysql_username)

instead of

mysql_connect($mysql_hostname , $mysql_username)

 

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in C:\xampp\htdocs\ .php:13
Stack trace: #0 C:\xampp\htdocs\chat\index.php(2): include() #1 {main} thrown in C:\xampp\htdocs\chat\config.php on line 13