Why is my Login Form Failing to Authenticate Users Despite No Apparent Errors?

Unable to Connect Login Form to MySQL Database

This question concerns an issue connecting a login form to a MySQL database. The user encounters no errors during login attempts, but the process fails to authenticate users. To resolve this issue, we'll delve into the provided login form and accompanying PHP script to debug the connection and authentication mechanisms.

The submitted code for loginpage.php appears straightforward and should capture user credentials, including username and password, via a form that posts data to login.php. However, the subsequent handling in login.php reveals potential problems.

Inside login.php, the code starts by assigning empty strings to both $error and $username variables, indicating an intent to manage error messages and username information. It's worth noting that this approach lacks basic input validation for these inputs. Insecure user input should always be sanitized and validated before being used in database queries or other operations.

Moving on, the if (empty($_POST['username']) || empty($_POST['password'])) condition checks if either the username or password input fields are empty. If so, the $error variable is set to "Username or Password is invalid," and the user is not authenticated.

Assuming both fields contain non-empty input, the code establishes a database connection using the provided parameters. However, the connection approach here lacks error handling. If any connection issues arise, they won't be caught and reported, making it difficult to debug the cause.

Next, it executes an SQL query to fetch registered user information and compare it to the provided credentials. The mysqli_query function returns false on failure, which should be checked. If the query fails, the script should terminate with an error message or log the issue for further examination.

The code subsequently counts the number of rows using mysqli_num_rows. But this check has a logical flaw: it expects exactly one row to exist for a valid login. However, in practice, it's possible to have duplicate records due to data inconsistencies or malicious attempts. It's wiser to check for the presence of at least one row instead of relying on an exact count.

If a row is found, the user is considered authorized, and the $_SESSION['login_user'] variable is set with the username. However, it's not clear how the code responds when multiple matching rows are found or how it manages potential race conditions or concurrency issues.

The code proceeds to close the database connection. However, if there's an error during any of the previous operations (such as database queries), the connection may be left open, causing resource leaks or performance issues.

To enhance the robustness and security of this code, additional measures could include:

• Using prepared statements with parameterized queries to prevent SQL injection attacks.
• Employing a more robust authentication mechanism, such as hashing and salting passwords, instead of storing passwords in cleartext.
• Adding error handling and logging throughout the script to identify and address connection problems or query failures.
• Enforcing unique constraints on the 'username' field in the database to prevent duplicate user accounts.

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