note 51806 added to function.header

I'm planning to do a simple authentication. I want to redirect the page to another php page file if the user name and password is correct. But. I'm having a problem with the header(). HERE is the part of my code that is showing the error. <body> <?php if ($p = $u) { header("Location:1.php"); //I'm getting the error below /*Warning: Cannot modify header information - headers already sent by (output started at C:\WebRoot\mySite\auth.php:3) in C:\WebRoot\mySite\auth.php on line 4*/ exit; } ?> </body> How can i solve this problem? ---- Manual Page -- http://www.php.net/manual/en/function.header.php Edit -- http://master.php.net/manage/user-notes.php?action=edit+51806 Delete: added to the manual -- http://master.php.net/manage/user-notes.php?action=delete+51806&report=yes&reason=added+to+the+manual Delete: bad code -- http://master.php.net/manage/user-notes.php?action=delete+51806&report=yes&reason=bad+code Delete: spam -- http://master.php.net/manage/user-notes.php?action=delete+51806&report=yes&reason=spam Delete: useless -- http://master.php.net/manage/user-notes.php?action=delete+51806&report=yes&reason=useless Delete: other reasons -- http://master.php.net/manage/user-notes.php?action=delete+51806&report=yes Reject -- http://master.php.net/manage/user-notes.php?action=reject+51806&report=yes Search -- http://master.php.net/manage/user-notes.php