PHP Errors: Notice: Undefined variable, index, offset and how to fix them

Hello everyone! I'm a beginner in PHP and constantly encounter various Notices and Warnings when working with variables and arrays. For example, I have this code:

<?php\n$user = $_POST['username'];\necho "Hello, " . $user;\n\n$data = array('name' => 'Ivan');\necho $data['age'];\n\n$items = array('apple', 'banana');\necho $items[5];\n\necho $undefined_var;\n?>

And I get in the browser:

\n
• Notice: Undefined variable: undefined_var
\n
• Notice: Undefined index: username
\n
• Warning: Undefined array key "age"
\n
• Notice: Undefined offset: 5
\n

The script works, but these messages spoil the output and look scary. I've read that you can disable display_error