Creates an initialization vector (IV) from a random source.
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 losing security.
The size of the IV.
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). Prior to 5.3.0, MCRYPT_RAND was the only one supported on Windows.
Note that the default value of this parameter was MCRYPT_DEV_RANDOM prior to PHP 5.6.0.
Note that MCRYPT_DEV_RANDOM may block until more entropy is available.
Returns the initialization vector, or false on error.