Hello all, working on a project now and found out when i test the site on my local system, the page redirected to the mainmenu.php when the logon on was successful, but after uploading the revserse was the case, the header() function didn't redirect to the mainmenu.php, can someone help

Sample code:
<?php
session_start();
// Call up the configration page and the web effect function page
require_once('connection/application_config1.php');
require_once("function/webeffect_function.php";
$uname = mysql_prep($_POST['u']);
$pass = mysql_prep($_POST['p']);
$addpassword = addpassword($uname);
$queryresult = checkusers($uname,$pass);
//check session to hold the application
//check session to hold the application
$_SESSION['fullname'] = $queryresult['f_name'];
$_SESSION['email'] = $queryresult['e_mail'];
$_SESSION['username'] = $queryresult['u_name'];
$mess_error = error_type(14);
{header("location:mainmenu.php?welcome_mess=".$mess_error['mess']);}
?>

<?php




if(isset($_POST['login']))
{

$uname = mysql_prep($_POST['u']);
$pass = mysql_prep($_POST['p']);

$sql = "SELECT `id`,`f_name`,`mobile` FROM users WHERE
           `u` = '$uname'
AND `p` = '$pass'
";


$result = mysql_query($sql) or die (mysql_error());
$num = mysql_num_rows($result);
  // Match row found with more than 1 results  - the user is authenticated.
    if ( $num > 0 ) {





     // this sets session and logs user in 
       
   session_start();
   // this sets variables in the session
$_SESSION['email']= $e_mail; 
$_SESSION['username'] = $u_name;
$_SESSION['fullname'] = $queryresult['f_name'];



   


header("Location: mainmenu.php";
}
else
{
$msg = urlencode("Please try again with correct user email and password. ";
header("Location: index.php?msg=$msg";
}

}



?>

Good Job MJ