Passwords and Authentication

Never store plain or reversibly encrypted passwords. Use password_hash() with PASSWORD_DEFAULT (bcrypt today) or PASSWORD_ARGON2ID, verify with password_verify() (constant-time), and call password_needs_rehash() after login to upgrade hashes when cost or algorithm changes. Rate-limit login attempts, return generic error messages, and regenerate the session ID after login.

<?php
$hash = password_hash($plainPassword, PASSWORD_ARGON2ID);

function login(UserRepository $users, string $email, #[\SensitiveParameter] string $password): ?User
{
    $user = $users->findByEmail($email);
    if ($user === null || !password_verify($password, $user->passwordHash)) {
        return null;                       // same response for both cases
    }
    if (password_needs_rehash($user->passwordHash, PASSWORD_ARGON2ID)) {
        $user->passwordHash = password_hash($password, PASSWORD_ARGON2ID);
        $users->save($user);
    }
    session_regenerate_id(true);
    $_SESSION['user_id'] = $user->id;
    return $user;
}

Common pitfalls

• Using md5()/sha1() or a single SHA-256 for passwords.
• Different error messages for unknown email vs wrong password (account enumeration).
• Not regenerating the session ID at login (session fixation).

Key points

• password_hash / password_verify
• Rehash when algorithms change
• Regenerate session IDs on login

Further reading: PHP manual: password_hash