index.php

<?php

  require 'config.php';

  if ( isset($_POST['submit']) ) {
    $username = mysqli_real_escape_string($connection, trim($_POST['username']) );
    $password = mysqli_real_escape_string($connection, trim($_POST['password']) );

    $sql = mysqli_query($connection, " SELECT username, password FROM user WHERE username='$username' ");
    list($username, $passwd) = mysqli_fetch_array($sql);
    $row = mysqli_num_rows($sql);
    if ($row > 0) {
      if ( password_verify($password, $passwd) ) {
        session_start();
        $_SESSION['username'] = "$username";
        $_SESSION['status']   = "login";
        header("Location:landing/home.php");
      } else {
        header("Location:index.php?message=login_failed");
      }

    } else {
      echo "Username doesn't exists!";
    }

  }

 ?>


<?php

  session_start();
  if (isset($_SESSION['status']) == "login") {
    header("Location:landing/home.php");
  }

 ?>


<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Login</title>
  </head>
  <body>
    <form action="" method="post">
      <div>
        <label>Username</label>
        <input type="text" name="username" placeholder="username" required>
      </div>
      <div>
        <label>Password</label>
        <input type="password" name="password" placeholder="password" required>
      </div>
      <div>
        <input type="submit" name="submit" value="Log In">
      </div>
    </form>
  </body>
</html>