[Nero_3D]

You should create a vehicle array

PHP код:

// global
new gVehicleOwner[MAX_VEHICLES + 1]; 


The code gets way more simply

PHP код:

public OnPlayerStateChange(playerid, newstate, oldstate) {
    if(newstate == PLAYER_STATE_DRIVER) {
        new
            vehicleid = GetPlayerVehicleID(playerid),
            owner = gVehicleOwner[vehicleid]
        ;
        if(owner == INVALID_PLAYER_ID) {
            // vehicle has no owner
        } else {
            if(owner == playerid) {
                // Vehicle is owned by playerid
            } else {
                // Vehicle isn't owned by playerid
                RemovePlayerFromVehicle(playerid);
            }
            new string[64];
            GetPlayerName(owner, string, MAX_PLAYER_NAME);
            format(string, sizeof(string), "This car is owned by %s", string);
            SendClientMessage(playerid, -1, string);
        }
    }
    return 1;
}