As of MySQL v5.0.3, MySQLs DECIMAL datatype stores an exact decimal number, i.e. not an inaccurate floating point representation.
To correctly manipulate precision numbers in PHP use the arbitrary precision math functions. Internally this library manipulates text strings.
Currency is intended to be stored as a decimal number, you can get units of money smaller than cents. You should only round any values when displaying the figures, not during manipulation or storage.