These constants don't cover all the possible options.
There are two constants:
These relate to using OFB mode in 8-bit feedback mode and using the same feedback length as the key
There is one constant
MCRYPT_MODE_CFB which uses 8-bit feedback mode
There ought to be a constant
MCRYPT_MODE_NCFB which uses the same feedback length as the key.
In fact you can just use the option 'ncfb' in mycrypt
Folgende Konstanten werden von dieser Erweiterung definiert und stehen nur zur Verfügung, wenn die Erweiterung entweder statisch in PHP kompiliert oder dynamisch zur Laufzeit geladen wurde.
Mcrypt can operate in four block cipher modes (CBC, OFB, CFB, and ECB). If linked against libmcrypt-2.4.x or higher the functions can also operate in the block cipher mode nOFB and in STREAM mode. Below you find a list with all supported encryption modes together with the constants that are defines for the encryption mode. For a more complete reference and discussion see Applied Cryptography by Schneier (ISBN 0-471-11709-9).
MCRYPT_MODE_ECB(electronic codebook) is suitable for random data, such as encrypting other keys. Since data there is short and random, the disadvantages of ECB have a favorable negative effect.
MCRYPT_MODE_CBC(cipher block chaining) is especially suitable for encrypting files where the security is increased over ECB significantly.
MCRYPT_MODE_CFB(cipher feedback) is the best mode for encrypting byte streams where single bytes must be encrypted.
MCRYPT_MODE_OFB(output feedback, in 8bit) is comparable to CFB, but can be used in applications where error propagation cannot be tolerated. It's insecure (because it operates in 8bit mode) so it is not recommended to use it.
MCRYPT_MODE_NOFB(output feedback, in nbit) is comparable to OFB, but more secure because it operates on the block size of the algorithm.
MCRYPT_MODE_STREAMis an extra mode to include some stream algorithms like "WAKE" or "RC4".
Some other mode and random device constants: