<?php
$data = json_decode(file_get_contents('php://input'), true);
if ($data['secret'] != "Wi39JHfg03Sjh3w03htj3T") {
    exit('error');
}
function message_send($id, $message)
{
    $arg = [
        'peer_id' => $id,
        'random_id' => mt_rand(0, 1000),
        'message' => $message,
        'v' => '5.90',
        'access_token' => 'c5641459f20520762df132c276a642e38acc131267391b2ca4a5fbe64e097f4ff7ea3883956c00231809c'
    ];
    file_get_contents('https://api.vk.com/method/messages.send?' . http_build_query($arg));
}

switch ($data['type']) {
    case 'confirmation' :
        exit('739a7370');
    case 'message_allow' :
        message_send($data['object']['user_id'], 'Привет, напиши help');
        break;
    case 'message_new':
        if ($data['object']['text'] == mb_strtolower('help')) {
            message_send($data['object']['from_id'], 'Бот умеет: складывать, вычетать, делить, умножать 2 числа');
            break;
        }
        $calc = trim($data['object']['text']);
        $expression = $calc;
        $first_number = '';
        $operator = '';
        $second_number = '';
        $pointer = 'first';
        $pos = '0';
        while (mb_strlen(mb_substr($expression, $pos, 1)) > 0) {
            $char = mb_substr($expression, $pos, 1);
            if ($pointer == 'first' && !is_numeric($char)) {
                $pointer = 'second';
                $operator = $char;
            } else {
                ${$pointer . '_number'} .= $char;
            }
        }
        $first_number = (int)$first_number;
        $second_number = (int)$second_number;
        if ($operator == '+') {
            message_send($data['object']['from_id'], $first_number + $second_number);
        } elseif ($operator == '-') {
            message_send($data['object']['from_id'], $first_number - $second_number);
        } elseif ($operator == '*') {
            message_send($data['object']['from_id'], $first_number * $second_number);
        } elseif ($operator == '/') {
            message_send($data['object']['from_id'], $first_number / $second_number);
        } else {
            echo 'Я тебя не понимаю';
        }
        break;
}

echo 'ok';
?>