header problem

hello I recentrly uploaded my project from localhost to a hosting and found many errors and warnings which didnt have in local. one of the most annoying one is header('Location xxx'). I have used header to redirect users from pages, and kinda used it alot. i know about the whitespace causing warning, but most of the pages i'm sending users got html and php mixed so i'm confused about how to remove whitespace in a html/php file. the error is : Warning: Cannot modify header information - headers already sent by .... here is a simple example, user update page :

    if($valid)
                   {
                       $msg='111';
                       $user->dbupdate();
                        header("Location: /admin/index.php?msg=$msg");

                   }
                   else
                   {
                        foreach($errors as $val)
                           echo '<p id="error">'.$val.'</p>';
                   }

and on admin index i get $msg and display it. <html> ...... //lots of stuff <?php

                  $msg = $_GET['msg'];
                          switch($msg){
                          case '111'       echo '<p> </p>';
                                       break;
                         case '421':...
                   }

?> // more html and php how can i fix this?