<?php
class Calculator
{
    /*
     * Class for calculating 2 numbers
     */
    var $expression;

    /*
     * Constructor takes 2 numbers and an operator
     * @param int $number Number to be converted
     * @param string $unit Conversion unit
     */
    public function __construct($expression = '')
    {
        $this->expression = $expression;
    }

    /*
     * Calculates number and returns result
     * @return string Result of calculation
     */
    public function calculate(): string
    {
        $result = false;
        $this->expression = preg_replace('/\s+/', '', $this->expression);
        if(preg_match('/(\d+)([x\*\-\+\/])(\d+)/', $this->expression, $matches)) {
            $number1 = $matches[1];
            $operator = $matches[2];
            $number2 = $matches[3];
            if (is_numeric($number1) && is_numeric($number2)) {
                switch ($operator) {
                    case '+':
                        $result = ($number1 + $number2);
                        break;
                    case '-':
                        $result = ($number1 - $number2);
                        break;
                    case '*':
                    case 'x':
                        $result = ($number1 * $number2);
                        break;
                    case '/':
                        $result = ($number1 / $number2);
                        break;
                }
            }
        } return $result;
    }
}