OOP Introduction tutorial in PHP

 <?php

class Calculation

{

    public $a,$b,$c;

    function sum()

    {

        $this->c = $this->a + $this->b;

        return $this->c;

    }

    function sub()

    {

        $this->c = $this->a - $this->b;

        return $this->c;

    }

}

$c1 = new Calculation();

$c2 = new Calculation();

echo "Value of a is : " . $c1->a = 20;

echo "<br>";

echo "Value of b is : " . $c1->b = 10;

echo "<br>";

echo "Sum is : " . $c1->sum() . "<br>";

echo "Subtraction is : " . $c1->sub() . "<br><br>";

echo "Value of a is : " . $c2->a = 50;

echo "<br>";

echo "Value of b is : " . $c2->b = 35;

echo "<br>";

echo "Sum is : " . $c2->sum() . "<br>";

echo "Subtraction is : " . $c2->sub() . "<br>";

?>