Really Basic Question about PHP includes

[cheat-master30]

Note: No scripting is being run using includes by myself, nor anyone else. This is on including plain HTML files, not programs/scripts/etc.

What is the difference between all these ways of writing includes?

On W3Schools, they use this code:

<?php include("menu.php"); ?>

PHP.net uses this code:

include 'vars.php';

And some books use this code:

<?php include "menu.php"; ?>

Some use brackets and speech marks, some quotation marks and some either speech marks or quotations marks without brackets. Is there a specific standard in this respect, or is it a matter of choice which varies from tutorial to tutorial?

The second thing is this. Why do many books go back on what they say about security and Super Globals in later examples?