LightestDev's login system

 <?php
 ini_set('display_errors', 1);
 ini_set('display_startup_errors', 1);
 error_reporting(E_ALL);

 include('includes/functions.php');
 if(isset($_POST['login'])) {

  //create variables
  //wrap the data with our function
  $formUser = validateFormData($_POST['username']);
  $formPass = validateFormData($_POST['password']);

  // connect to database
  include('includes/connection.php');

  //create SQL query
  $query = "SELECT username, email, password FROM users WHERE username='$formUser'";

  // store the result
  $result = mysqli_query($conn, $query);

  // verify if result is returned
  if (mysqli_num_rows($result) > 0 ) {
    // store basic user data in variables
    while ($row = mysqli_fetch_assoc($result)) {
      $user = $row["username"];
      $email = $row["email"];
      $hashedPass = $row["password"];
  }

  // verify hashed password with the typed password
  if (password_verify($formPass, $hashedPass)) {

    // correct login details!
    // start the Session
    session_start();

    //store data in session variables
    $_SESSION['loggedInUser'] = $user;
    $_SESSION['loggedInEmail'] = $email;

    //Even though there aren't any errors, we're gonna initialize the variable
    $loginError = "";

    header("Location: checker.php");
  } else {
    // error message
    $loginError = "<div class='alert alert-danger'>Please check your username or password.</div>";
  }

  }else {
    // there are no results in database
    $loginError = "<div class='alert alert-danger'>No user in the database. <a class='close' data-dismiss='alert'>&times</a></div>";
  }

  // close mysql connection
  mysqli_close($conn);

 }

 ?>


 <!DOCTYPE html>
 <html lang="en">
 <head>
   <title>Login</title>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
   <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
   <script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
 </head>
 <body>

 <div class="container">
   <h1>Login</h1>
   <p class="lead">Log in to your account!</p>

   <?php echo $loginError ?>

   <form class="" action="<?php echo htmlspecialchars($_SERVER['PHP_SELF'] ); ?>" method="post">
 <div class="form-group">
     <label>* Username</label>
     <input type="text" name="username" value="Username" class="form-control" id="login-username" placeholder="username">
 </div>
 <div class="form-group">
     <label>* Password</label>
     <input type="password" name="password" value="Password" class="form-control" id="login-password" placeholder="password">
 </div>
 <button type="submit" class="btn btn-default" name="login">Login</button>
 </form>
 </div>

 </body>
 </html>