User Login Script

<?php
	if(isset($_POST['g-recaptcha-response']) && !empty($_POST['g-recaptcha-response'])):
        //your site secret key
        $secret = '6LfqLSMTAAAAABFcgM8H4ViMHT2WKsKegH8A8bkB';
        //get verify response data
        $verifyResponse = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret='.$secret.'&response='.$_POST['g-recaptcha-response']);
        $responseData = json_decode($verifyResponse);
        if($responseData->success):

    // First we execute our common code to connection to the database and start the session
    require("common.php");

	if(isset($_SESSION['user']))
    {
        // If they are not, we redirect them to the login page.
        header("Location: ../dashboard");

        // Remember that this die statement is absolutely critical.  Without it,
        // people can view your members-only content without logging in.
        die("You are already logged in! Redirecting to Dashboard...");
    }

    // This variable will be used to re-display the user's username to them in the
    // login form if they fail to enter the correct password.  It is initialized here
    // to an empty value, which will be shown if the user has not submitted the form.
    $submitted_username = '';

    // This if statement checks to determine whether the login form has been submitted
    // If it has, then the login code is run, otherwise the form is displayed
    if(!empty($_POST))
    {

        // This query retreives the user's information from the database using
        // their username.
        $query = "
            SELECT
                id,
                username,
                password,
                salt,
                email
            FROM users
            WHERE
                username = :username
        ";

        // The parameter values
        $query_params = array(
            ':username' => $_POST['username']
        );

        try
        {
            // Execute the query against the database
            $stmt = $db->prepare($query);
            $result = $stmt->execute($query_params);
        }
        catch(PDOException $ex)
        {
            // Note: On a production website, you should not output $ex->getMessage().
            // It may provide an attacker with helpful information about your code.
            die("Failed to run query: " . $ex->getMessage());
        }

        // This variable tells us whether the user has successfully logged in or not.
        // We initialize it to false, assuming they have not.
        // If we determine that they have entered the right details, then we switch it to true.
        $login_ok = false;

        // Retrieve the user data from the database.  If $row is false, then the username
        // they entered is not registered.
        $row = $stmt->fetch();
        if($row)
        {
            // Using the password submitted by the user and the salt stored in the database,
            // we now check to see whether the passwords match by hashing the submitted password
            // and comparing it to the hashed version already stored in the database.
            $check_password = hash('sha256', $_POST['password'] . $row['salt']);
            for($round = 0; $round < 65536; $round++)
            {
                $check_password = hash('sha256', $check_password . $row['salt']);
            }

            if($check_password === $row['password'])
            {
                // If they do, then we flip this to true
                $login_ok = true;
            }
        }

        // If the user logged in successfully, then we send them to the private members-only page
        // Otherwise, we display a login failed message and show the login form again
        if($login_ok)
        {
            // Here I am preparing to store the $row array into the $_SESSION by
            // removing the salt and password values from it.  Although $_SESSION is
            // stored on the server-side, there is no reason to store sensitive values
            // in it unless you have to.  Thus, it is best practice to remove these
            // sensitive values first.
            unset($row['salt']);
            unset($row['password']);

            // This stores the user's data into the session at the index 'user'.
            // We will check this index on the private members-only page to determine whether
            // or not the user is logged in.  We can also use it to retrieve
            // the user's details.
            $_SESSION['user'] = $row;

			// This creates a log of the login action.

			$query = "
            INSERT INTO logs (
                type,
                user,
                ip
            ) VALUES (
                :type,
                :user,
                :ip
            )
			";

			// Here we prepare our tokens for insertion into the SQL query.
			$query_params = array(
				':type' => "User Login",
				':user' => $_POST['username'],
				':ip' => $_SERVER['REMOTE_ADDR']
			);

			try
			{
				// Execute the query to create the blog post
				$stmt = $db->prepare($query);
				$result = $stmt->execute($query_params);
			}
			catch(PDOException $ex)
			{
				// Note: On a production website, you should not output $ex->getMessage().
				// It may provide an attacker with helpful information about your code.
				die("Failed to run query: " . $ex->getMessage());
			}

            // Redirect the user to the Dashboard page.
            header("Location: ../dashboard");
            die("Redirecting to Dashboard...");
        }
        else
        {
            // Tell the user they failed
            header("Location: ../login.php?badlogin");
			die("Login Failed - Please try again...");

            // Show them their username again so all they have to do is enter a new
            // password.  The use of htmlentities prevents XSS attacks.  You should
            // always use htmlentities on user submitted values before displaying them
            // to any users (including the user that submitted them).  For more information:
            // http://en.wikipedia.org/wiki/XSS_attack
            $submitted_username = htmlentities($_POST['username'], ENT_QUOTES, 'UTF-8');
        }
	}
		endif;
		endif;

		header("Location: ../login.php?badlogin");
		die("The Recaptcha was entered incorrectly.");
?>