(PHP 5 >= PHP 5.3.3)
openssl_cipher_iv_length — Gets the cipher iv length
$method
   )Gets the cipher iv length.
methodThe method.
   Returns the cipher length on success, or FALSE on failure. 
  
   Emits an E_WARNING level error when the cipher algorithm
   is unknown.
  
Example #1 openssl_cipher_iv_length() example
<?php
$method = 'AES-128-CBC';
$ivlen = openssl_cipher_iv_length($method);
echo $ivlen;
?>
The above example will output something similar to:
16