What are some common reasons for receiving the "Undefined index" error in PHP?

The "Undefined index" error in PHP occurs when trying to access an array key that does not exist. This can happen if the array key is not set or if there is a typo in the key name. To solve this issue, you should always check if the index exists before trying to access it using isset() or array_key_exists() functions. 

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

Keywords

PHP Undefined index Error Array Associative array

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