login_update.php

<?php
include "db.php";
include "functions.php";

/* show errors in console */
  ini_set('display_errors', 1);
  ini_set('display_startup_errors', 1);
  error_reporting(E_ALL);

  // connection needs to be global
        global $connection;
        $query = "SELECT * FROM tl_users";
        // Performs a query on the database (connection, query)
        $result = mysqli_query($connection, $query);


  // error: , instead of &&
  if(isset($_POST['username'], $_POST['password']) ){
    $username = $_POST['username'];
    $password = $_POST['password'];
  }

?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css">
</head>
<body>
<div class="container">

    <div class="col-sm-6">

        <form action="login_update.php" method="post">
            <div class="form-group">
            <label for="username">Username</label>
            <!-- display current username -->
            <input type="text" name="username" class="form-control" value="<?php echo $username;?>">
            </div>

            <!-- display current password (change password to text) -->
            <div class="form-group">
            <label for="password">Password</label>
            <input type="text" name="password" class="form-control" value="<?php echo $password;?>">
            </div>

            <!-- id field -->
            <div class="form-group">
            <select name="" id="">
                <?php
                    showAllData();
                ?>
            </select>
            </div>

            <input class="btn btn-primary" type="submit" name="submit" value="Update">

        </form>


    </div>
</div>
</body>
</html>