how do I display the value of a variable in headinclude template?

[pompaunpo]

this is my plugin hook at global_start (but I also tried to global_setup_complete)

function AD_Open($type) {
$result_AD=$db->query_first("SELECT css from mytable WHERE id ='$type'");
$csscode = $result_AD['css'];
return $csscode;
}

$dom_css = AD_Open(2);

in headinclude template I put $dom_css but is empty
why?