(PHP 3>= 3.0.8, PHP 4 , PHP 5)
mcrypt_create_iv -- Create an initialization vector (IV) from a random sourcemcrypt_create_iv() is used to create an IV.
mcrypt_create_iv() takes two arguments, size determines the size of the IV, source specifies the source of the IV.
The source can be MCRYPT_RAND (system random number generator), MCRYPT_DEV_RANDOM (read data from /dev/random) and MCRYPT_DEV_URANDOM (read data from /dev/urandom). If you use MCRYPT_RAND, make sure to call srand() before to initialize the random number generator. MCRYPT_RAND is the only one supported on Windows because Windows (of course) doesn't have /dev/random or /dev/urandom.
The IV is only meant to give an alternative seed to the encryption routines. This IV does not need to be secret at all, though it can be desirable. You even can send it along with your ciphertext without loosing security.
More information can be found at http://www.ciphersbyritter.com/GLOSSARY.HTM#IV, http://fn2.freenet.edmonton.ab.ca/~jsavard/crypto/co0409.htm and in chapter 9.3 of Applied Cryptography by Schneier (ISBN 0-471-11709-9) for a discussion of this topic.