<?php

class Calc
{
    public $result;

    public function __construct($string)
    {
        $this->parse($string);
    }

    protected function parse($string)
    {
        while (true) {
            preg_match('/\(([^()]*)\)/', $string, $matches);
            if (!count($matches)) {
                break;
            }
            $replace = $this->calculate($matches[1]);
            $string = str_replace($matches[0], $replace, $string);
        }
        $this->result = $this->calculate($string);
    }

    protected function calculate($string)
    {
        $operations = ['*' => 'mul', '/' => 'div'];

        foreach ($operations as $operation => $method) {
            while (true) {
                preg_match('/(-?\d+) \\' . $operation . ' (-?\d+)/', $string, $matches);
                if (!count($matches)) {
                    break;
                }
                $string = $this->$method($string, $matches);
            }
        }

        while (true) {
            preg_match('/(-?\d+) (\+|-) (-?\d+)/', $string, $matches);
            if (!count($matches)) {
                break;
            }
            if ($matches[2] == '+') {
                $string = $this->sum($string, $matches);
            } else {
                $string = $this->sub($string, $matches);
            }
        }

        return $string;
    }

    protected function mul($string, $matches)
    {
        return str_replace($matches[0], $matches[1] * $matches[2], $string);
    }

    protected function div($string, $matches)
    {
        return str_replace($matches[0], $matches[1] / $matches[2], $string);
    }

    protected function sum($string, $matches)
    {
        return str_replace($matches[0], $matches[1] + $matches[3], $string);
    }

    protected function sub($string, $matches)
    {
        var_dump($matches);
        return str_replace($matches[0], $matches[1] - $matches[3], $string);
    }
}

$calc = new Calc('3 * 5 + 7 * 2');
var_dump('3 * 5 + 7 * 2 = ' . $calc->result);

$calc = new Calc('5 * (7 * (5 + 3) / 4 - 4)');
var_dump('5 * (7 * (5 + 3) / 4 - 4) = ' . $calc->result);

$calc = new Calc('(2 * (2 * 2 * (2 * 2)))');
var_dump('(2 * (2 * 2 * (2 * 2))) = ' . $calc->result);

$calc = new Calc('-5 + (3 * (-2 - 4))');
var_dump('-5 + (3 * (-2 - 4)) = ' . $calc->result);