I realized that there is no query string at all. paypal simply forward to the redirect URL.

I started using sandbox. I made sure that auto return is enabled. and also PDT is enabled. still I am redirected to a plain URL.
finally I read this article. and found it bit useful.
http://blog.aktech.co.uk/post/2011/03/10/PHP-PayPal-Sandbox-Teturn-URL-is-missing-tx-ID-value-in-querystring.aspx

DONT FORGET TO VERIFY YOUR EMAIL

So to verify your emailID you go to profile->Add/Edit Email. then click on Confirm button. this will send a mail.

and now the bit tricky part about sandbox paypal

sandbox  it never send mail to your emailID so don’t open your gmail/yahoo inbox. u wont find it there.

for that you need to login to developer.paypal.com and click on test mail and see the mail there.