下列常量由此扩展定义，且仅在此扩展编译入 PHP 或在运行时动态载入时可用。
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: