if your using PHP, then i wouldn't recommend using mySQL to encrypt your passwords. PHP includes a function that will allow you to encrypt passwords through it. you can however use mySQL to encrypt functions, but then you must use mySQL to check the validity of a password as well.
to check if a password is correct, you must encrypt the submission, then check if it is the same as the encrypted data in the database. I believe it's something like this
$result = mysql_query("SELECT FROM members WHERE username='" . $user . "' AND password=PASSWORD('" $pass . "');");
that should work, but it really depends on what ur wanting to use passwords for, and how your database is setup.