public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger) //This is called wheneven a player tries to enter a vehicle, by press F or G. (Or whatever your defaults for passenger/driver are.)
{
    if(ispassenger == 1) //if ispassenger equals 1, this means they are entering as a passenger, by pressing G (default)
    {
        //Now that we know the player is entering as a passenger, we can do the following:
        SetPlayerPos(playerid, X, Y, Z); //The players position gets set to where the interior of the vehicle is made
        SetPlayerInterior(playerid, interiorid); //Sometimes if the interior is in another interior, you use this to set the players interior to it
        //SetPlayerVirtualWorld(playerid, worldid); //This is also sometimes used, so the player cannot be seen by other players who are not also in the vehicle.
    }
    return 1;
}

public OnPlayerStateChange(playerid, newstate, oldstate) //This is called when the state of the player changes, like entering the vehicle and becoming a driver, as a new state
{
    if(newstate == PLAYER_STATE_PASSENGER) //If the new state is a passenger
    {
        if(oldstate == PLAYER_STATE_ONFOOT) //if they used to be on foot before becoming a passenger
        {
            SetPlayerPos(playerid, X, Y, Z);
            SetPlayerInterior(playerid, interiorid);
            SetPlayerVirtualWorld(playerid, worldid);
        }
    }
    return 1;
}