Pass value from parent to child window in php

parent.php <?php $userid=37; echo "<form><input name=\"$userid\" align=\"center\" class=\"button btn btn-primary\" type=\"button\" onclick=\"window.open('popup.php?id='+$userid,'windowfrm','menubar=yes,toolbar=yes,scrollbars=yes,resizable=yes,width=783,height=auto')\" value=\"Printable Version\"></div>\n</form>\n\n"; ?> popup.php <?php $userid = $_GET['id']; echo $userid; ?>