Conditional redirect based on use case from work.
Page a.php is moving to b.php, and b.php is not yet ready, but for some reason (well, we'll assume there is a reason) it is necessary to direct users to b.php to tell them that the new content will be hosted there, and then send them back to the original content at a.php without redirecting them again to b.php.
This is one way to do it in php. We introduce a query string parameter on the request to a.php that - if present - will force the content to load with 200 response; otherwise it 302 redirects to b.php. When b.php is completely ready it will be turned into a permanent 301 redirect.
(In reality a.php and b.php live on different domains but this example adapts easily to that case - and you could replace the query string condition with something else, like a referal domain check or other.)