php - output of calculation is undesired -

I'm trying to get basic information based on the definition of class and use the calculation < P> Code here

  & lt ;? Php class calculator {var $ number1 = 4; Var $ number2 = 5; Add function ($ a, $ b) {$ c = $ a + $ b; Print ("The sum of your numbers: $ c"); Print ($ c); }} $ Cal = new calculator; $ Cal- & gt; Add ($ number 1, $ number 2); ? & Gt;   
 What's visible in my browser:  
 
 The sum of your numbers: 0   
 why not 9 < What are the values ​​of Code> $ number1  and  $ number2?   
 
 
  Are you passing?  $ number1  and  $ number2  as  $ cal-> Number1  and  $ cal- & gt; Number2 is not the same as .  
 You are defining the two properties of an object, and passing two separate, different variables in the function of the class. You are basically two pairs of numbers - with a pair of objects, values ​​of 4 and 5, and there is no value outside the function (both 0) that you are adding.  
 You can try this:  
    
 Or this:  
  & lt ;? Php class calculator {Private $ number1 = 4; Private $ number2 = 5; Add function () {$ c = $ this- & gt; Number 1 + $ this- & gt; Number 2; Print ("The sum of your numbers: $ c"); Print ($ c); }} $ Cal = new calculator; $ Cal- & gt; Add ();   
 Or this:  
  & lt ;? Php class calculator {add function ($ a, $ b) {$ c = $ a + $ b; Print ("The sum of your numbers: $ c"); Print ($ c); }} $ Cal = new calculator; $ Cal- & gt; Add (4, 5);