Upgrading Weak Password Hashes

Old apps store md5($password) or sha1($salt . $password). You can't reverse them, but you can migrate on login: verify with the legacy algorithm, then rehash with password_hash() and store it. For inactive users, wrap the old hash immediately: store password_hash(md5_hash) so weak hashes never sit in the database, and verify with password_verify(md5($input), $stored).

<?php
function verifyAndUpgrade(PDO $pdo, array $user, string $password): bool
{
    $hash = $user['password_hash'];
    if (strpos($hash, 'legacy-md5:') === 0) {                    // wrapped legacy hash
        $ok = password_verify(md5($password), substr($hash, 11));
    } else {
        $ok = password_verify($password, $hash);
    }
    if ($ok && (strpos($hash, 'legacy-md5:') === 0 || password_needs_rehash($hash, PASSWORD_DEFAULT))) {
        $pdo->prepare('UPDATE users SET password_hash = ? WHERE id = ?')
            ->execute([password_hash($password, PASSWORD_DEFAULT), $user['id']]);
    }
    return $ok;
}
// One-off migration for all rows:
// UPDATE: password_hash = 'legacy-md5:' . password_hash(old_md5, PASSWORD_DEFAULT)

Common pitfalls

• Leaving raw md5 hashes in the database until users log in.
• Comparing hashes with == (type juggling with '0e...' strings).
• Using PHP 8-only helpers like str_starts_with() in code that must still run on PHP 7.

Key points

• Wrap weak hashes immediately
• Upgrade to password_hash on login
• Never compare hashes with ==

Further reading: OWASP Password Storage Cheat Sheet