What are common causes of "Undefined variable" and "Fatal error" messages in PHP, specifically in the context of a login system?

Common causes of "Undefined variable" and "Fatal error" messages in PHP, particularly in a login system, include not initializing variables before using them, typos in variable names, and scope issues. To solve these errors, make sure to declare and initialize variables before using them, double-check variable names for accuracy, and ensure variables are within the correct scope.

// Example code snippet to prevent "Undefined variable" and "Fatal error" messages in a login system

// Initialize variables
$username = "";
$password = "";

// Check if form is submitted
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Retrieve input values
    $username = $_POST["username"];
    $password = $_POST["password"];
    
    // Add your login logic here
}