public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
if(GetPlayerWeapon(playerid) == 29)
    {
    //The players Enters the vehicle with MP5
    }else{
    /*You got 2 options [1] to ResetThePlayer Weapons
    //[2] to givehim wep id (0) witch will se the player holding no weapon
    Option [1]*/
    ResetPlayerWeapons(playerid);
    //Option[2]
    GivePlayerWeapon(playerid, 0, 1);
    }
    return 1;
}

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)// OnPlayerEnterVehicle callback, gets called when a player enters a vehicle.
{
    if(GetPlayerWeapon(playerid) == 29)// If player has MP5
    {
        return 1;// Returns value ( nothing. )
    }
    else// What happens if he doesn't have a MP5
    {
        SendClientMessage(playerid, 0xAA3333AA, " You canno't drive-by with a desert eagle !");// Sending a message
        GivePlayerWeapon(playerid, 0, 1); // Giving him fist, so he won't be able to use it.
        return 1;// Returning value for SendClientMessage
    }
    return 1; // Returning value, a must have.
}