login_update.php

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

<?php

if(isset($_POST['submit'])) {
global $connection;
$username = $_POST['username'];
$password = $_POST['password'];
$id = $_POST['id'];

$query = "UPDATE users SET ";
$query .= "username = '$username', ";
$query .= "password = '$password', ";
$query .= "WHERE id = '$id', ";

    $result = mysqli_query($connection, $query);
    if(!$result) {

        die("QUERY FAILED MAN" . mysqli_error($connection));

    }
}

?>


<!DOCTYPE HTML>
<html lang="en-US">
<head>
    <meta charset="UTF-8">
    <title></title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
</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>
        <input type="text" name="username" class="form-control">
        </div>

        <div class="form-group">
        <label for="password">Password</label>
        <input type="password" name="password" class="form-control">
        </div>

        <div class="form-group">

        <select name="id" id="">

    <?php

        showAllData();

    ?>


        </select>

        </div>

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


    </form>


</div>

</div>


</body>
</html>