Login.php

<?php
    session_start();

    if(isset($_POST['login'])) {
        include_once("db.php");
        $username = strip_tags($_POST['username']);
        $password = strip_tags($_POST['password']);

        $username = stripslashes($username);
        $password = stripslashes($password);

        $username = mysqli_real_escape_string($db, $username);
        $password = mysqli_real_escape_string($db, $password);

        $password = md5($password);

        $sql = "SELECT * FROM users WHERE username='$username' LIMIT 1";
        $query = mysqli_query($db, $sql);
        $row = mysqli_fetch_array($query);
        $id = $row['id'];
        $db_password = $row['password'];

        if($password == $db_password) {
            $_SESSION['username'] = $username;
            $_SESSION['id'] = $id;
            header("Location: index.php");
        } else {
   		header("Location: access_denied.php");
   	}
   }


?>

<!doctype html>
<html>
<head>
	<title>Login</title>


</head>

<body>
	<h1>Login</h1>

	<form action="login.php" method="POST" enctype="multipart/form-data">
	    <label>Username:<br/></label>
	    <input type="text" name="username" required="required" autofocus><br/>
	    <label>Password:<br/></label>
	    <input type="password" name="password" required="required"><br/><br/>
        <input type="submit" value="Login">
	</form>

	<br/>

	<a>Not yet registered? </a><a href="register.php">Click Here</a>
</body>

</html>