尝试此代码

$vehicles = DB::table('vehicles')
                    ->orderBy('categoryname', 'asc')
                    ->get();
    $cat = "";
    foreach($vehicles AS $vehicle) {
        if($vehicle->categoryname != $cat) {
            $cat = $vehicle->categoryname;
            echo $cat . '<br><br>';
        }
        echo $vehicle->brandname . '<br>';
    }

$vehicles = DB::table('vehicles')->get();
$arr = [];
foreach($vehicles as $veh)
{
    if(isset( $arr[$veh->categoryname] ))
    {
       $arr[$veh->categoryname][] = $veh->brandname;
    } else 
    {
       $arr[$veh->categoryname][] = $veh->categoryname;
    }     
}
dd($arr);