public OnPlayerCommandText(playerid, cmdtext[])
{
    if (strcmp("/destroycar", cmdtext, true, 10) == 0)
    {
        for(new v = 0; v < MAX_VEHICLES; v++)
        {
            GetClosestVehicle(v);
            DestroyVehicle(v);
        }
        return 1;
    }
    return 0;
}

stock GetClosestVehicle(playerid, &Float:dis = (Float:0x7F800000))
{
  dis = (Float:0x7F800000);
  new Float:X, Float:Y, Float:Z;
  if(GetPlayerPos(playerid, X, Y, Z)) {
    new vehicleid = INVALID_VEHICLE_ID;
    for(new v, Float:temp, Float:VX, Float:VY, Float:VZ; v != MAX_VEHICLES; v++) {
      if(GetVehiclePos(v, VX, VY, VZ)) {
        VX -= X, VY -= Y, VZ -= Z;
        temp = VX * VX + VY * VY + VZ * VZ;
        if(temp < dis) dis = temp, vehicleid = v;
      }
    }
    dis = floatpower(dis, 0.5);
    return vehicleid;
  }
  return INVALID_VEHICLE_ID;
}