What best practices should be followed when handling user input and authentication in PHP, especially when dealing with sensitive data like passwords?

When handling user input and authentication in PHP, it is crucial to sanitize and validate user input to prevent SQL injection and cross-site scripting attacks. When dealing with sensitive data like passwords, always hash the passwords using a strong hashing algorithm like bcrypt before storing them in the database. Additionally, use prepared statements when interacting with the database to prevent SQL injection vulnerabilities. 

// Sanitize and validate user input
$username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
$password = filter_var($_POST['password'], FILTER_SANITIZE_STRING);

// Hash the password before storing it in the database
$hashed_password = password_hash($password, PASSWORD_DEFAULT);

// Use prepared statements to interact with the database
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = ?");
$stmt->execute([$username]);
$user = $stmt->fetch();

if ($user && password_verify($password, $user['password'])) {
    // Authentication successful
    // Proceed with user actions
} else {
    // Authentication failed
    // Handle error or redirect to login page
}

Keywords

SQL injection password hashing prepared statements password_verify secure sessions

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