PHP不输出数组

Question

所以我想尝试一些后端样式代码，而ive则用php弄乱了一些。我编写了一个简单的程序（使用html和php），要求用户创建一个帐户，然后将存储这些用户名和密码。

我在底部添加了一个功能，询问用户是否要显示特定的用户名（un）和密码（pw）。但是，当我输入一些随机的un和pw并键入1（因此程序应输出un 1和pw 1）时，它将无法工作...

非常感谢您的帮助！

<html>
  <body>
    <style>
      input 
        font-family: 'Trebuchet MS'
      
      form 
        font-family: 'Trebuchet MS'
      
      body 
        background-color: lightblue

      
      h1 
        font-family: 'Trebuchet MS'

      
    </style>
    <h1 id = "signup_here">Signup here!</h1>
    <form action = "[the website uses my real name, so im not including it]" method = "get">
      Your Username: <input type = "text" name = "username">
      <br/>
      Your Password: <input type = "text" name = "password">
      <input type = "submit">
    </form>
    <br/>
    <?PHP
      $username = $_GET["username"];
      $password = $_GET["password"];
      if(!is_null($username) and !is_null($password))
        echo "<h3>Your username is [$username] and your password is [$password]";
      
    ?>
    <form method = "post">
      Please confirm: <input type = "checkbox" name = "up_conf" value = "val1">
      <input type = "submit">
    </form>
    <?PHP
      if(isset($_POST["up_conf"])) 
        echo "<h3>Thanks for signing up!</h3>";
      
    ?>
    <?PHP
      if(isset($_POST["up_conf"])) 
        $allusernames = array($_GET["username"]);
        $allpasswords = array($_GET["password"]);
      
    ?>
    <form action = "https://PhP-Playground.saatvikrk.repl.co" method = "get">
      Get usernames and passwords? 1/2/3/etc.: <input type = "text" name = "gaup">
      <?PHP
        if(!is_null($_GET["gaup"])) 
          echo "Username:" . $allusernames[$_GET["gaup"]];
          echo "Password:" . $allpassword[$_GET["gaup"]];
        
      ?>
    </form>
  </body>
</html>