Cannot modify header information – headers already sent by by (output started a

I added this code to the top header.php file (note: there are no blank spaces before the code)

<?
// if the user doesn’t have a cookie set, take them to the splash page
if(!isset($_COOKIE[‘no_splash’])&&((strpos($_SERVER[‘HTTP_REFERER’],get_bloginfo(‘home’))===false)&&!$_SERVER[‘QUERY_STRING’])) {
header(‘Location:https://myurl.php&#8217;); // Redirect code
} else { ?>

and I tried both with and without a closing tag at the end of the page.

this is the code I have on my splash page:

<?// Set the expiration time to 1 day – 60 seconds * 60 minutes * 24 hours * 1 day
// You can change the (1) to any number of days you like
$expire = time() + 60*60*60*360;

// set the cookie using the above expiration time
setcookie(“no_splash”, “1”, $expire);

//contents here
?>

The splash page is in WordPress Install Folder