Task: 21 game
C++: 

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iomanip>
#include <limits>

using namespace std;
#define _(STR) STR


class Model
{
public:
    static const int GOAL = 21;
    static const int NUMBER_OF_PLAYERS = 2;
    static const int MIN_MOVE = 1;
    static const int MAX_MOVE = 3;
    int bestMove();
    bool update(const char* player, int move);
    bool isGameBegin();
    bool isGameOver();
protected:
    friend class View;
    View* view = nullptr;
    const char* player = nullptr;
    int oldTotal = 0;
    int newTotal = 0;
    int lastMove = 0;
};


class View
{
protected:
    Model* model;
public:
    View(Model* model);
    void init(const char* player);
    void update();
};


class Controller
{
protected:
    Model* model;
    View* view;
public:
    Controller(Model* model, View* view);
    int input();
    void clear();
    void run();
};


int Model::bestMove()
{
    
    

    int move = MIN_MOVE;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if ((newTotal + i - 1) % (MAX_MOVE + 1) == 0)
            move = i;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if (newTotal + i == GOAL)
            move = i;
    return move;
}

bool Model::update(const char* player, int move)
{
    if (move >= MIN_MOVE && move <= MAX_MOVE && newTotal + move <= GOAL)
    {
        this->player = player;
        oldTotal = newTotal;
        newTotal = oldTotal + move;
        lastMove = move;
        view->update();
        return true;
    }
    else
        return false;
}

bool Model::isGameBegin()
{
    return oldTotal == 0;
}

bool Model::isGameOver()
{
    return newTotal == GOAL;
}


View::View(Model* model)
{
    this->model = model;
    model->view = this;
}

void View::init(const char* player)
{
    if (model->newTotal == 0)
        cout << _("----NEW GAME----\n\n")
             << _("The running total is currently zero.\n")
             << _("The first move is ") << player << _(" move.\n\n");
}

void View::update()
{
    cout << setw(8) << model->player << ": " << model->newTotal << " = "
         << model->oldTotal << " + " << model->lastMove << endl << endl;
    if (model->isGameOver())
        cout << endl << _("The winner is ") << model->player << _(".\n\n\n");
}


Controller::Controller(Model* model, View* view)
{
    this->model = model;
    this->view = view;
}

void Controller::run()
{
    if (rand() % Model::NUMBER_OF_PLAYERS == 0)
    {
        view->init("AI");
        model->update("AI", model->bestMove());
    }
    else
        view->init("human");

    while (!model->isGameOver())
    {
        while (!model->update("human", input()))
            clear();
        model->update("AI", model->bestMove());
    }
}

int Controller::input()
{
    int value;
    cout << _("enter a valid number to play (or enter 0 to exit game): ");
    cin >> value;
    cout << endl;
    if (!cin.fail())
    {
        if (value == 0)
            exit(EXIT_SUCCESS);
        else
            return value;
    }
    else
        return model->MIN_MOVE - 1;
}

void Controller::clear()
{
    cout << _("Your answer is not a valid choice.") << endl;
    cin.clear();
    cin.ignore((streamsize)numeric_limits<int>::max, '\n');
}


int main(int argc, char* argv)
{
    srand(time(NULL));

    cout << _(
        "21 Game                                                          \n"
        "                                                                 \n"
        "21 is a two player game, the game is played by choosing a number \n"
        "(1, 2, or 3) to be added to the running total. The game is won by\n"
        "the player whose chosen number causes the running total to reach \n"
        "exactly 21. The running total starts at zero.                    \n\n");

    while (true)
    {
        Model* model = new Model();
        View* view = new View(model);
        Controller* controler = new  Controller(model, view);

        controler->run();

        delete controler;
        delete model;
        delete view;
    }
    return EXIT_SUCCESS; 
}

PHP: <!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="keywords"    content="Game 21">
    <meta name="description" content="
      21 is a two player game, the game is played by choosing a number
      (1, 2, or 3) to be added to the running total. The game is won by
      the player whose chosen number causes the running total to reach
      exactly 21. The running total starts at zero.
    ">
    <!--DCMI metadata (Dublin Core Metadata Initiative)-->
    <meta name="dc.publisher" content="Rosseta Code">
    <meta name="dc.date"      content="2020-07-31">
    <meta name="dc.created"   content="2020-07-31">
    <meta name="dc.modified"  content="2020-08-01">
    <title>
        21 Game
    </title>
    <!-- Remove the line below in the final/production version. -->
    <meta http-equiv="cache-control" content="no-cache">

    <style>
      .ui div   { width: 50%; display: inline-flex; justify-content: flex-end; }
      div.total { margin-bottom: 1ch; }
      label     { padding-right: 1ch; }
      button + button { margin-left: 1em; }
    </style>
</head>

<body>

  <h1>
    21 Game in PHP 7
  </h1>

  <p>
    21 is a two player game, the game is played by choosing a number
    (1, 2, or 3) to be added to the running total. The game is won by
    the player whose chosen number causes the running total to reach
    exactly 21. The running total starts at zero.
  </p>


  <?php

    const GOAL = 21;
    const PLAYERS = array('AI', 'human');

    function best_move($n)
    {
      for ($i = 1; $i <= 3; $i++)
        if ($n + $i == GOAL)
          return $i;
      for ($i = 1; $i <= 3; $i++)
        if (($n + $i - 1) % 4 == 0)
          return $i;
      return 1;
    }

    if (isset($_GET['reset']) || !isset($_GET['total']))
    {
      $first = PLAYERS[rand(0, 1)];
      $total = 0;
      $human = 0;
      $ai = 0;
      $message = '';
      if ($first == 'AI')
      {
        $move = best_move($total);
        $ai = $move;
        $total = $total + $move;
      }
    }
    else
    {
      $first   = $_GET['first'];
      $total   = $_GET['total'];
      $human   = $_GET['human'];
      $ai      = $_GET['ai'];
      $message = $_GET['message'];
    }

    if (isset($_GET['move']))
    {
      $move = (int)$_GET['move'];
      $human = $move;
      $total = $total + $move;
      if ($total == GOAL)
        $message = 'The winner is human.';
      else
      {
        $move = best_move($total);
        $ai = $move;
        $total = $total + $move;
        if ($total == GOAL)
          $message = 'The winner is AI.';
      }
    }

    $state = array();
    for ($i = 1; $i <= 3; $i++)
      $state[$i] = $total + $i > GOAL ? 'disabled' : '';

    echo <<< END
      <p>
        The first player is $first.
        Use buttons to play.
      </p>
      <form class="ui">
        <div>
          <input type='hidden' id='first' name='first' value='$first'>
          <input type='hidden' name='message' value='$message'>
        </div>
        <div class='total'>
          <label for='human'>human last choice:</label>
          <input type='text' name='human' readonly value='$human'>
        </div>
        <div class='total'>
          <label for='AI'>AI last choice:</label>
          <input type='text' name='ai' readonly value='$ai'>
        </div>
        <div class='total'>
          <label for='runningTotalText'>running total:</label>
          <input type='text' name='total' readonly value='$total'>
        </div>
        <div class='buttons'>
          <button type='submit' name='move' value='1' {$state[1]}> one   </button>
          <button type='submit' name='move' value='2' {$state[2]}> two   </button>
          <button type='submit' name='move' value='3' {$state[3]}> three </button>
          <button type='submit' name='reset' value='reset'> reset </button>
        </div>
      </form>
      <p>
        $message
      </p>
    END
  ?>

</body>