sum of 2 number with object and method

Question

why am I getting these errors? Undefined variable: num1 Cannot access empty property

class sum{
    public $num1=1;
public function fun($num2){
     return $this->$num1+$num2;
    }
}
$number = new sum();
echo $number->fun(3);

Answer 1

class sum{
   public $num1=1;
   public function fun($num2){
        return $this->num1+$num2;
          //removed $^^
   }
}
$number = new sum();
echo $number->fun(3);