php - Cannot modify header information - headers already sent message in my browser

//The register-process page is displaying the following message when my RegistrationPage.php:

Warning: Cannot modify header information - headers already sent by (output started at /home/cs12jkk/public_html/validation.inc:4) in /home/cs12jkk/public_html/register-process.php on line 67

//Here is the code on line 67:

//final disposition
if (count($_SESSION['error']) > 0) {
    die (header("Location: RegistrationPage.php"));
} else {
    if(registerUser($_POST)) {
        unset($_SESSION['formAttempt']);
        die(header("Location: success.php"));

} else {
error_log("Problem registering user: {$_POST['email']}");
        $_SESSION['error'][] = "Problem registering account";
        die(header("Location: RegistrationPage.php"));
    }
} 

//this is my validation.inc code:

function is_valid_($county) {
    $validCounties = array ( "Avon","Bedfordshire","Berkshire","Borders","Buckinghamshire","Cambridgeshire","Central","Cheshire","Cleveland","Clwyd","Cornwall","County Antrim","County Armagh","County Down","County Fermanagh","County Londonderry","County Tyrone","Cumbria","Derbyshire","Devon","Dorset","Dumfries and Galloway","Durham","Dyfed","East Sussex","Essex","Fife","Gloucestershire","Grampian","Greater Manchester","Gwent","Gwynedd County","Hampshire","Herefordshire","Hertfordshire","Highlands and Islands","Humberside","Isle of Wight","Kent","Lancashire","Leicestershire","Lincolnshire","Lothian","Merseyside","Mid Glamorgan","Norfolk","North Yorkshire","Northamptonshire","Northumberland","Nottinghamshire","Oxfordshire","Powys","Rutland","Shropshire","Somerset","South Glamorgan","South Yorkshire","Staffordshire","Strathclyde","Suffolk","Surrey","Teesside","Tyne and Wear","Warwickshire","West Glamorgan","West Midlands","West Sussex","West Yorkshire","Wiltshire","Worcestershire",);
    if (in_array($county,$validCounties)) {
            return true;
    } else {
            return false;
    }
} //end function is_valid_county


function is_valid_postcode($postcode) {
        if (preg_match('/^[\d]+$/',$postcode)) {
                return true;
        } else if (strlen($postcode) == 5 || strlen($postcode) == 9) {
                return true;
        } else {
                return false;
        }
} //end function is_valid_postcode


?>

//any idea what I'm doing wrong?