Very useful article..
A Guide to Cryptography in PHP
by Octavia Andreea Anghel
For every difficult and complicated question there is an answer that is simple, easily understood, and wrong. H.L. Mencken
string md5(string $str [, bool $raw_output ])
one-way encryption function.. useful for passwords..
string crypt (string $str [, string $salt ])
string sha1 (string $str [, bool $raw_output ])
MCrypt MHash Crypt_Blowfish Crypt_RSA Crypt_HMAC Crypt_DiffieHellman