A login system looks simple on the surface — check a username and password, start a session — but there are several places where beginners (and experienced developers) accidentally introduce serious vulnerabilities. Here's how to do it properly.

First, never store plain-text passwords. Use PHP's built-in password_hash() with the default bcrypt algorithm when a user registers, and password_verify() to check the password at login. Never write your own hashing scheme — the built-in functions handle salting and are battle-tested.

Second, use prepared statements for every query that touches user input. String-concatenating a username into a SQL query is the single most common way login systems get compromised. PDO with bound parameters closes this door completely and costs nothing extra to implement.

Third, protect against brute-force attempts. Track failed login attempts per account (or per IP) and introduce a short lockout or delay after several failures. This doesn't need to be complex — even a simple attempts counter in the database with a time-based cooldown meaningfully raises the bar for attackers.

Fourth, handle sessions carefully. Regenerate the session ID after a successful login (session_regenerate_id()) to prevent session fixation, set a reasonable session timeout, and always use HTTPS in production so session cookies can't be intercepted over the network.

Fifth, validate and sanitize on the way in, and escape on the way out. Validate that an email looks like an email before hitting the database; escape any user-supplied content with htmlspecialchars() before echoing it back into HTML to prevent stored XSS.

Finally, add CSRF protection to your login and any other form that changes state — a hidden token tied to the session, checked on submit. It's a small addition that closes a whole class of cross-site attacks. None of this is exotic; it's a checklist, and following it consistently is what separates a login system that's fine in a demo from one that's safe in production.