What is the significance of the "Undefined index" notice in PHP?

The "Undefined index" notice in PHP occurs when trying to access an array key that does not exist. This can be solved by checking if the index exists before trying to access it, using the isset() function. This helps prevent errors and ensures that the code runs smoothly without any notices being displayed to the user.

if(isset($_POST['submit'])) {
    $value = isset($_POST['key']) ? $_POST['key'] : '';
}