Please Pardon if the question sounds silly, but nevertheless its a question which I want to know. :)
How can I redirects which display that you are being redirected (like older Gmail and LinkedIn). Whenever I tried to do that, I got errors saying that Headers were already sent.
Then somebody here told me that I should not output any markup before redirection (like facebook login). So how do I go about it and display something nice during redirection??
Those redirections are not done via "normal" redirection HTTP headers. Instead they display a page and use either a META Refresh or some Javascript to navigate to the new page.
Personally I find both methods not very nice, both for users and for search engines. Using HTTP headers that also signify why there is a redirect (Permanently moved, temporary, &c.) are way better imho.
you want to use meta redirects. they enable you to show a page, and after a few seconds this page will send you to the new page. all you have to do is add a tag to the portion of your 'something nice' redirection page.
here's a quick tutorial on meta redirects: http://webdesign.about.com/od/metataglibraries/a/aa080300a.htm
you want to do something like this:
<html>
<head>
<meta http-equiv="refresh" content="2;url=whereto">
were 2 is the number of seconds to display your page and whereto is where you want to send your user
You need to output your page, which will include the following META keyword:
<html>
<head>
<!-- redirection to example.com in 5 seconds -->
<meta http-equiv="refresh" content="5;url=http://example.com/" />
...
</head>
<body>
...
Read the following article for more help: http://en.wikipedia.org/wiki/Meta_refresh
If you are getting the headers already sent then that means you were trying to redirect with a PHP header() redirect and you had output on the screen before calling the header() function, 1 solution to that is to use PHP's Output Buffering which will then allow you to call the header() redirect anywhere on the page and not get that error.
To show a message you could use the meta method mentioned in some other answers maybe even throw in a javascript redirect with it, just to be safe however the way I would do it would be something like this below
<?PHP
ob_start();
//show a message
echo 'You will be redirected in 10 seconds';
//amount of time to show the message above before redirecting
sleep(200);
//redirect
header('http://domain.com');
?>
I did not test this but you should get the idea, actually now that I think about it, sleep() might not work correctly as expected with output buffering