What is the significance of the "Undefined index" notice in PHP and how can it be avoided?

The "Undefined index" notice in PHP occurs when trying to access an array key that does not exist. To avoid this notice, you can check if the key exists before trying to access it using isset() or array_key_exists() functions. Example PHP code snippet: 

// Check if the key exists before accessing it
if(isset($_POST['username'])) {
    $username = $_POST['username'];
    // Use $username variable here
} else {
    // Handle the case when 'username' key is not set
}

Keywords

PHP Undefined index Notice Error handling Array indexing

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