How can the use of register globals impact the functionality of PHP scripts, and what are the best practices for handling this issue?

Using register globals can impact the functionality of PHP scripts by making variables easily accessible from outside sources, leading to security vulnerabilities like injection attacks. The best practice for handling this issue is to disable register globals in the PHP configuration or to explicitly initialize variables using the $_POST, $_GET, or $_REQUEST superglobals.

// Disable register globals in php.ini
// Or explicitly initialize variables using superglobals
$var = isset($_POST['var']) ? $_POST['var'] : null;