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

You can use mysqli_connect($mysql_hostname , $mysql_username) instead of mysql_connect($mysql_hostname , $mysql_username).

mysql_* functions were removed as of PHP 7. You now have two alternatives: MySQLi and PDO.

It is recommended to use either the MySQLi or PDO extensions. It is not recommended to use the old mysql extension for new development, as it was deprecated in PHP 5.5.0 and was removed in PHP 7.

PHP offers three different APIs to connect to MySQL. Below we show the APIs provided by the mysql, mysqli, and PDO extensions. Each code snippet creates a connection to a MySQL server running on "example.com" using the username "username" and the password "password". And a query is run to greet the user.

Example #1 Comparing the three MySQL APIs

<?php
// mysqli
$mysqli = new mysqli("example.com", "username", "password", "database");
$result = $mysqli->query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = $result->fetch_assoc();
echo htmlentities($row['_message']);

// PDO
$pdo = new PDO('mysql:host=example.com;dbname=database', 'username', 'password');
$statement = $pdo->query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = $statement->fetch(PDO::FETCH_ASSOC);
echo htmlentities($row['_message']);

// mysql
$c = mysql_connect("example.com", "username", "password");
mysql_select_db("database");
$result = mysql_query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = mysql_fetch_assoc($result);
echo htmlentities($row['_message']);
?>

I suggest you try out both MySQLi and PDO and find out what API design you prefer.

Read Choosing an API and Why shouldn't I use mysql_* functions in PHP?

As other answers suggest... Some guy (for whatever reason) decided that your old code should not work when you upgrade your PHP, because he knows better than you and don't care about what your code does or how simple it is for you to upgrade.

Well, if you can't upgrade your project overnight you can

downgrade your version of PHP to whatever version that worked

or...

use a shim (kind of polyfill) such as https://github.com/dshafik/php7-mysql-shim or https://github.com/dotpointer/mysql-shim, and then find a place for include_once("choice_shim.php"); somewhere in your code

That will keep your old PHP code up and running until you are in a mood to update...

mysql_* functions have been removed in PHP 7.

You probably have PHP 7 in XAMPP. You now have two alternatives: MySQLi and PDO.