Self-Redirect With Adding Parameters in PHP

Let the GET parameter named arg be passed to the page index.php. Let's display its content on the screen:

<?php
	echo $_GET['arg'];
?>

Let's assume our page cannot work correctly without the GET parameter. In our case, this is indeed true, because if the parameter is not passed, then accessing $_GET['arg'] will cause an error.

The question may arise, why would a person end up on our page without the parameter at all? After all, we can put links with this parameter everywhere on our site. However, this is not a guarantee: a user of our site might, for example, copy a link from somewhere and lose the parameter in the process. Or simply accidentally delete it while editing the address bar.

In any case, our code should anticipate such a situation and do something about it. What can be done?

We can check our GET parameter for existence and output it only if it exists:

<?php
	if (isset($_GET['arg'])) {
		echo $_GET['arg'];
	} else {
		// react somehow, for example, with a message
	}
?>

We can act more cleverly:

<?php
	if (!isset($_GET['arg'])) {
		$_GET['arg'] = 'default'; // default value
	}
	
	echo $_GET['arg']; // will guaranteed output something without error
?>

Or we can make it so that when accessing the page without a parameter, a redirect to the same page with the parameter occurs:

<?php
	if (!isset($_GET['arg'])) {
		header('Location: ?arg=default');
	}
	
	echo $_GET['arg']; // parameter is guaranteed to exist
?>

Explain the difference between the second option and the third one.

Let the number be passed to the page page.php using a GET parameter named num. Make it so that when accessing without this parameter, a redirect to the same page is automatically performed, but with the parameter num set to the value 1.