How can PHP global variables affect the functionality of form submissions?

PHP global variables can affect the functionality of form submissions by allowing data to be easily accessed and manipulated across different parts of the code. This can lead to security vulnerabilities if the global variables are not properly sanitized or validated before being used in form submissions. To mitigate this issue, it is recommended to avoid using global variables for storing form data and instead use superglobal arrays like $_POST or $_GET to retrieve form data securely.

// Example of securely handling form submissions using superglobal arrays
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = htmlspecialchars($_POST["username"]);
    $password = htmlspecialchars($_POST["password"]);
    
    // Validate and process form data securely
}