Are there any best practices or recommendations to prevent the occurrence of "Undefined index" errors in PHP scripts, especially when handling form submissions?

When handling form submissions in PHP, "Undefined index" errors occur when trying to access an array key that does not exist. To prevent these errors, always check if the key exists before accessing it using isset() or array_key_exists(). This helps ensure that you are only trying to access keys that actually exist in the array.

if(isset($_POST['submit'])) {
    if(isset($_POST['username'])) {
        $username = $_POST['username'];
        // process username
    }
    if(isset($_POST['password'])) {
        $password = $_POST['password'];
        // process password
    }
}