How can the use of REGISTER GLOBALS impact the functionality of PHP code?

Using REGISTER GLOBALS can impact the functionality of PHP code by making variables easily accessible and modifiable from external sources, leading to security vulnerabilities and unpredictable behavior. To solve this issue, you should avoid using REGISTER GLOBALS and instead use the $_GET, $_POST, and $_SESSION superglobals to handle input data securely.

// Avoid using REGISTER GLOBALS
$var = $_POST['var'];