personal-website/docs/www/login.php

<?php
$COMMONS = $_SERVER['DOCUMENT_ROOT'] . "/../common";

include_once($COMMONS."/header.php");
include_once($COMMONS."/utils.php");

display_header("Login");

// Define previous attempt and error variables and set to empty values.
$emailOld = $passwordOld = "";
$emailErr = $passwordErr = "";

/**
 * Process the information, and if there are no errors, log the user in.
 */
function attempt_login($email, $password) {
	// Access global variables
	global $emailOld;
	global $passwordOld;
	global $emailErr;
	global $passwordErr;
	global $conn;

	// Check DB connection
	if($conn == null){
		printf("
			<article>
				<h1>Failed DB connection, cannot proceed!</h1>
				If you see this error in production,
				please shoot me an email with helpful details.
			</article>");
		include_once($GLOBALS['COMMONS']."/footer.php");
		die();
	}

	// Sanitize inputs
	$email = sanitize_input($email);
	$password = sanitize_input($password);

	// Check if both fields are filled, if not, set appropriate error messages.
	if (empty($email))
		$emailErr = "Please enter your email";
	if (empty($password))
		$passwordErr = "Please enter your password";

	// If either of the fields were empty,
	// set old values for prefill and return.
	if(!empty($emailErr) || !empty($passwordErr)) {
		$emailOld = $email;
		$passwordOld = $password;
		return;
	}

	// Prepare and bind the sql statement
	$stmt = $conn->prepare("SELECT user_id, username, email, password
		FROM users WHERE email = :email;");
	$stmt->bindParam(":email", $email);

	// Execute the statement
	$stmt->execute();

	// Fetch the values
	$result = $stmt->fetch(PDO::FETCH_ASSOC);

	// If the user isn't in the database, set errors, old values, and return.
	if(!$result) {
		$emailOld = $email;
		$passwordOld = $password;
		$emailErr = "This user either doesn't exist,
					or has a different password.";
		return;
	}

	// Load results to variables
	$db_id = $result["user_id"];
	$db_username = $result["username"];
	$db_email = $result["email"];
	$db_password = $result["password"];

	// If user entered incorrect password, set errors, old values, and return.
	// Keep the error string the same as non-existing so that an attacker
	// cannot asses whether a given user has an account.
	if(!password_verify($password, $db_password)){
		$emailOld = $email;
		$passwordOld = $password;
		$emailErr = "This user either doesn't exist,
					or has a different password.";
		return;
	}

	$_SESSION["user_id"] = $db_id;
	$_SESSION["user_name"] = $db_username;
	$_SESSION["user_email"] = $db_email;
}

/**
 * If user sent the form, process it.
 * Either login user or set error message variables.
 */
if ($_SERVER["REQUEST_METHOD"] == "POST") {
	attempt_login($_POST["email"], $_POST["password"]);
}
?>

<article>
	<form method="post" action="<?php
		echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>">
		<h2> Login: </h2>
		<table class="noborder-table"><tr>
			<td> Email: </td>
			<td>
				<input type="text" name="email" tabindex="1"
				autofocus="autofocus" value="<?php echo $emailOld;?>">
			</td>
			<td>
				<?php echo $emailErr; ?>
			</td>
		</tr><tr>
			<td> Password: </td>
			<td>
				<input type="password" name="password" tabindex="2"
				value="<?php echo $passwordOld?>">
			</td><td>
				<?php echo $passwordErr; ?>
			</td>
		</tr></table>
		<input type="submit" tabindex="3" value="Send">
	</form>
</article>

<?php
include_once($COMMONS."/footer.php");
?>