[=WoR=G4M3Ov3r]

PHP код:

CMD:trailer(playerid, params[])
{
    new vehicleid;
    vehicleid = GetPlayerVehicleID(playerid);
    if(IsATruck(vehicleid))
    {
        new randtrailer = random( sizeof( Trailers ) );
        AttachTrailerToVehicle(randtrailer, vehicleid);
        SendClientMessage(playerid, COLOR_RED, "INFO: Trailer attached");
        printf("%d", randtrailer);
    }
    return 1;
}
stock IsATruck(vehicleid)
{
    switch(GetVehicleModel(vehicleid))
    {
        case 435, 450, 584, 591: return 1;
    }
    return 1;
} 


Try this

Edit : Late, Thanks to the Anti-Spam System ^^.