I am new to oop. I just can't debug this simple code. I don't know why the arguments don't get passed through it.

```
class MyCalculator {  
    private $_fval, $_sval;  

    public function add($fval, $sval) {  
        return $this->_fval + $this->_sval;  
    }  

    public function subtract($fval, $sval) {  
        return $this->_fval - $this->_sval;  
    }  

    public function multiply($fval, $sval) {  
        return $this->_fval * $this->_sval;  
    }  

    public function divide($fval, $sval) {  
        return $this->_fval / $this->_sval;  
    }  
}  

$mycalc = new MyCalculator;   
echo $mycalc-> add(12,6); 
echo $mycalc-> multiply(12,6); 
echo $mycalc-> subtract(12,6);  
echo $mycalc-> divide(12,6); 

```