Optozooi - untested.

class OptoInput
{
public:
  OptoInput(int pin, char up, char down)
  {
    Pin = pin;
    upchar = up;
    downchar = down;
    pinMode(pin, INPUT);
  }

  void Update()
  {
    int newval = digitalRead(Pin);
    if (newval != lastval)
    {
      if (newval)
      {
        Serial.write(upchar);
      }
      else
      {
        Serial.write(downchar);
      }
      lastval = newval;
    }

  }
  char upchar, downchar;
  int lastval;
  int Pin;
};

OptoInput Opto5(5,'5', 't');
OptoInput Opto6(6,'6', 'y');
OptoInput Opto7(7,'7', 'u');
OptoInput Opto8(8,'8', 'i');

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  Opto5.Update();
  Opto6.Update();
  Opto7.Update();
  Opto8.Update();
}