IP based redirect

Hi all, I wrote this redirect function which sort of works

function underConstruction()
{
    $visitor = $_SERVER['REMOTE_ADDR'];
    $valid_ips = array("1","2","3","4");

    if(!in_array($visitor, $valid_ips))
    {
        header("Location: http://www.redirecthere.com");
    }
}

I’ve removed the IP addresses. Now this…kind of works. It works if I try to access any other page of the site other than the home page from a non valid IP. When accessing the home page, I get an error stating that the headers cannot be modified as they’ve already been sent. Anyone else ever had this issue?