header.tpl questions

Hi,

I have a header that I use in my website outside whmcs portal template,
I want to call the header just like I do to any page in my website to the portal template and when I add the following line to header.tpl nothing is showing:

<?include_once"http://www.mydomain.com/myheader.php";?>

what could be the problem not letting my header show in header.tpl

Thanks in advance for your help,