string mhash ( int hash, string data [, string key] )
mhash() applies a hash function specified by
hash to the data and
returns the resulting hash (also called digest). If the key
is specified it will return the resulting HMAC. HMAC is keyed hashing
for message authentication, or simply a message digest that depends on
the specified key. Not all algorithms supported in mhash can be used in
HMAC mode. In case of an error returns FALSE.
