How can the "Notice: Undefined index" error be avoided in PHP scripts?

The "Notice: Undefined index" error in PHP scripts occurs when trying to access an array key that doesn't exist. To avoid this error, you should always check if the index/key exists before trying to access it using isset() or array_key_exists() functions. Example PHP code snippet: 

if(isset($_POST['submit'])) {
    if(isset($_POST['username'])) {
        $username = $_POST['username'];
        // process the username
    } else {
        echo "Username is not set";
    }
} else {
    echo "Submit button is not set";
}

Keywords

PHP error handling isset array_key_exists strict typing

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