Warning: Cannot modify header information - headers already sent by (output started at /home/pushpamsanskruti/public_html/admin/logout.php:3) in /home/pushpamsanskruti/public_html/admin/logout.php on line 5

This is because you've outputted something (probably tex) to the browser before your headers were sent. Headers net to be set and sent before you output text to the screen, because the headers usually tell the browser what kind of stuff it is that you are sending to the screen, and it uses that information to determine how to display that what you're outputting.

adding to minitauros answer above
text can be:
blank space, carriage return, line feed, crap code added by poor editors
the last can be hard to find, the editor may not display stuff it adds