can't send messages and $username isn't displayed-CodePudding

I made a chat service in php. When you get to the chat, it says "Welcome, --Username--." My problem is I added a login field, now I can't send messages and the username isn't displayed. I've looked everywhere but it seems since my code is so "unique", nothing seems to be helping. --i didn't make this all on my own, I used someone else's code for the login and someone's code for the service itself.

login.php

<?php
    session_start();
    echo isset($_SESSION['login']);
    if(isset($_SESSION['login'])) {
      header('LOCATION:admin.php'); die();
    }
?>
<!DOCTYPE html>
<html>
   <head>
     <meta http-equiv='content-type' content='text/html;charset=utf-8' />
     <title>Login</title>
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
   </head>
<body>
  <div class="container">
    <h3 class="text-center">Login</h3>
    <?php
      if(isset($_POST['submit'])){
        $username = $_POST['username']; $password = $_POST['password'];
        if($username === 'admin' && $password === 'password'){
          $_SESSION['login'] = true; header('LOCATION:admin.php'); die();
        } {
          echo "<div class='alert alert-danger'>Username and Password do not match.</div>";
        }
        if($username === 'admon' && $password === 'password'){
            $_SESSION['login'] = true; header('LOCATION:admin.php'); die();
          } {
            echo "<div class='alert alert-danger'>Username and Password do not match.</div>";
          }
          
      }
    ?>
    <form action="" method="post">
      <div class="form-group">
        <label for="username">Username:</label>
        <input type="text" class="form-control" id="username" name="username" required>
      </div>
      <div class="form-group">
        <label for="pwd">Password:</label>
        <input type="password" class="form-control" id="pwd" name="password" required>
      </div>
      <button type="submit" name="submit" class="btn btn-default">Login</button>
    </form>
  </div>
</body>
</html>

admin.php

<?php
    session_start();
    if(!isset($_SESSION['login'])) {
        header('LOCATION:login.php'); die();
    }

    if(isset($_GET['logout'])){    
     
        //Simple exit message
        $logout_message = "<div class='msgln'><span class='left-info'>User <b class='user-name-left'>". $_SESSION['name'] ."</b> has left the chat session.</span><br></div>";
        file_put_contents("log.html", $logout_message, FILE_APPEND | LOCK_EX);
         
        session_destroy();
        header("Location: login.php"); //Redirect the user
    }

?>
<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8" />
 
        <title>English Pricks</title>
        <meta name="description" content="A Group Chat." />
        <link rel="stylesheet" href="style.css" />
    </head>
    <body>
        <div id="wrapper">
            <div id="menu">
                <p class="welcome">Welcome, <b><?php echo $username['username']; ?></b>&period; &nbsp;<a href="https://docs.google.com/document/d/1S2O4y2z_8Yu_mibQQGEU4E9pHdeckPfo5pI7FtM0YrI/edit" target="_blank">Image Dump&period;&period;&period;</a></p>
                <p>Emoji's &#8594;&nbsp;</p>
                <button id="emoji-button" style="border: none;">