<?php

/**
 * 12.05.2015 (8:21)
 * Calculator.php
 * PhpStorm
 */

error_reporting(-1);
header("Content-Type: text/plain; charset=utf-8");
mb_internal_encoding('utf-8');

$input = '243+6743-78*2*1*1*1*1*1+4-3-3817/9=';
$inputLength = mb_strlen($input);

$number = 0;
$result = 0;
$op = '';

echo "Пример: {$input}\n\n";

for ($i = 0; $i < $inputLength; $i++) {

    $char = mb_substr($input, $i, 1);

    if (is_numeric($char)) {
        $number = ($number * 10) + intval($char);
    } elseif ($char == '*' || $char == '+' || $char == '-' || $char == '=' || $char == '/') {
        echo "op=[{$op}], result=[{$result}], number=[{$number}], char=[{$char}]\n";
        if ($op == '') {
            $result = $number;
        }
        if ($op == '*') {
        $result *= $number;
        }
        if ($op == '+') {
            $result += $number;
        }
        if ($op == '-') {
            $result -= $number;
        }
        if ($op == '/') {
            $result /= $number;
        }
        $number = 0;
        $op = $char;
    }
}

echo "\nОтвет: {$result}\n";