Historically, password security in PHP has been a bit slippery, requiring a measure of knowledge and care. Aiming to change that, PHP 5.5 introduces a special password_hash() function which makes password security much easier, and with features such as automatic algorithm upgrading, even more robust. There's also a compatibility library for PHP >= 5.3.7.
If you've ever looked at login code, the chances are you've seen developers using hash('sha256', $password), or even md5($password) to "secure" user passwords. Password hashes generated this way are laughably easy to crack; with weak algorithms and no salting or stretching in place you're almost giving your passwords to an attacker who gains access.