为什么mcrypt_encrypt会出错？

Question

I got the code below from this site. When I run it, I get

Warning: mcrypt_encrypt(): Attempt to use an empty IV, which is NOT recommend in C:\web\apache\htdocs\dev\encrypt.php on line 7

What wrong am I doing here and how do I get this right?

//Key

$key = 'SuperSecretKey';

//To Encrypt:
$encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, 'I want to encrypt this', MCRYPT_MODE_CFB);

//To Decrypt:
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CFB);

echo $encrypted;
echo "<br>";
echo $decrypted;

Answer 1

It is not error its warning, it clearly explained in Documentation

IV - Used for the initialization in CBC, CFB, OFB modes, and in some algorithms in STREAM mode. If you do not supply an IV, while it is needed for an algorithm, the function issues a warning and uses an IV with all its bytes set to "\0".