如何返回静态变量PHP

function build_path($cid)
{
    static $fr=array();
    $DB = new MySQLTable;
    $DB->TblName = 'shop_categories';
    $where['cat_id']['='] = $cid;
    $res = $DB->Select('cat_id,cat_name,cat_parent', $where);
    if($res !== false)
    {
        $pid = mysql_fetch_array($res);
        if($pid['cat_parent'] !== "0")
        {
           $fr[] = $pid['cat_id'];
           build_path($pid['cat_parent']);
        } else {
            $fr[] = $cid;
            $fr = array_reverse($fr);
            print_r($fr);
            return $fr;
        }
    }
}

print_r(build_path(100));

为什么在函数中使用print\r,但second print\r返回NULL?