Task: 21 game
VB: 

Class MainWindow
    Private Const GOAL As Integer = 21
    Private total As Integer = 0
    Private random As New Random

    Private Sub Update(box As TextBox, player As String, move As Integer)
        total = total + move
        box.Text = move
        boxTotal.Text = total
        If total + 1 > GOAL Then button1.IsEnabled = False
        If total + 2 > GOAL Then button2.IsEnabled = False
        If total + 3 > GOAL Then button3.IsEnabled = False
        If total = GOAL Then
            winner.Content = $"The winner is {player}."
        End If
    End Sub

    Private Sub Ai()
        Dim move As Integer = 1
        For i = 1 To 3
            If (total + i - 1) Mod 4 = 0 Then move = i
        Next i
        For i = 1 To 3
            If total + i = GOAL Then move = i
        Next i
        Update(boxAI, "AI", move)
    End Sub

    Private Sub Choice(sender As Object, e As RoutedEventArgs) _
            Handles button1.Click, button2.Click, button3.Click
        Update(boxHuman, "human", sender.Content)
        If total < GOAL Then Ai()
    End Sub

    
    
    
    Private Sub StartGame(sender As Object, e As RoutedEventArgs) Handles restart.Click

        total = 0

        boxAI.Text = ""
        boxHuman.Text = ""
        boxTotal.Text = ""
        
        winner.Content = ""

        button1.IsEnabled = True
        button2.IsEnabled = True
        button3.IsEnabled = True

        
        
        
        If random.Next(2) = 0 Then
            first.Content = "First player is AI player."
            Ai()
        Else
            first.Content = "First player is human player."
        End If
    End Sub
End Class

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>