What are the potential issues when upgrading from PHP 5.6.x to PHP 7, especially in terms of deprecated functions like mysql_connect?

When upgrading from PHP 5.6.x to PHP 7, one potential issue is the use of deprecated functions like `mysql_connect`. To solve this issue, you should switch to using the MySQLi or PDO extension for database connections, as `mysql_connect` has been removed in PHP 7.

// Deprecated way using mysql_connect
$connection = mysql_connect('localhost', 'username', 'password');

// Updated way using MySQLi
$connection = mysqli_connect('localhost', 'username', 'password');

// Updated way using PDO
$connection = new PDO('mysql:host=localhost;dbname=database', 'username', 'password');