PHP: how should I hash passwords?

Question

An old project stores passwords with `md5($password)`. What should I use instead, and how do I migrate existing users?

Accepted Answer

Use PHP's built-in password API. It picks a strong algorithm (bcrypt by default, Argon2id available) and handles salts for you:

```php
$hash = password_hash($password, PASSWORD_DEFAULT);
// or: password_hash($password, PASSWORD_ARGON2ID);

if (password_verify($input, $hash)) { /* ok */ }
```

**Migrating from MD5 without forcing resets:**
1. Immediately wrap existing hashes: store `password_hash(md5_hash, ...)` and mark those rows as `legacy`.
2. At login, for legacy rows verify `password_verify(md5($input), $stored)`. On success, re-hash with `password_hash($input, ...)` and clear the flag.
3. Also use `password_needs_rehash()` on every login so hashes upgrade automatically when you raise the cost.

MD5 hashes can be cracked in seconds, so do step 1 today.

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