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);