What are common pitfalls when migrating from PHP 5.3 to PHP 7.3 on an IIS server?

One common pitfall when migrating from PHP 5.3 to PHP 7.3 on an IIS server is the use of deprecated functions and features in PHP 5.3 that are no longer supported in PHP 7.3. To solve this issue, you will need to update your code to use the latest PHP functions and syntax supported in PHP 7.3.

// Before PHP 5.3
mysql_connect("localhost", "username", "password");

// After PHP 5.3
$mysqli = new mysqli("localhost", "username", "password");