OOP practice

<?php

class   problem {

    private $operator;
    private $num1;
    private $num2;
    private $solution;

    public function __construct($operator, $num1, $num2) {
        $this->operator = $operator;
        $this->num1 = $num1;
        $this->num2 = $num2;
    }

    function problemSolve() {
        switch ($this->operator){
            case '+':
                $this->solution = $this->num1 + $this->num2;
                break;
            case '-':
                $this->solution = $this->num1 - $this->num2;
                break;
            case '/':
                $this->solution = $this->num1 / $this->num2;
                break;
            case '*':
                $this->solution = $this->num1 * $this->num2;
                break;
            case 'sqrt':
                $this->solution = sqrt($this->num1);
                break;
            default:
                echo "Error, input not recognised.";
                break;
        }
    }

    function solutionOutput() {
        echo "$this->solution";
    }
}

$calc = new problem('put_operator_here', first_number_here, second_number_here);
$calc->problemSolve();
$calc->solutionOutput();