The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
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 defined 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_STREAM is an extra mode to include
     some stream algorithms like "WAKE" or "RC4".
    
   Some other mode and random device constants: