I'm not terribly familiar with MCRYPT, however, at a quick glance these appear to be two-way encryption algorithm functions. MD5, on the other hand, is a one-way algorithm (and consequently a function in PHP). One-way encryption algorithms are known as hashing, or taking a given amount of text and converting it into a set-length amount unique cyphertext that cannot be reversed.
MD5 hashing is a good technique to use for storing passwords in your database for example. You can use the built-in MD5 function in MySQL to hash the username (optionally more fields), with the real password to store a one-way encrypted value. Then upon login, you would re-hash this information and compare the results. If they are the same, the user can be authenticated.
Tell me specifically what you're trying to do so I can be more specific in answers.
