What is the correct format for a blowfish salt using PHP's crypt?

Question

I have read the information provided on the PHP Manual Entry for crypt(), but I find myself still unsure of the format for a salt to trigger the Blowfish algorithm.

According manual entry, I should use '$2$' or '$2a$' as the start of a 16 character string. However, in the example given later, they use a much longer string: '$2a$07$usesomesillystringforsalt$', which indicates to me that whatever string I provide will be sliced and diced to fit the model.

The problem I am encountering is actually triggering the Blowfish algo vs STD_DES. Example:

$foo = 'foo';
$salt = '$2a$' . hash('whirlpool', $foo); // 128 characters, will be truncated
$hash = crypt($foo, $salt); 
// $hash = $26HdMTpoODt6

That hash is obviously not whirlpool, and is in fact STD_DES with only the first two characters of the salt being used for the salt. However, in the PHP Manual's example, their salt starts with '$2a$07$', so if I add those three characters to the same code I get the following:

$foo = 'foo';
$salt = '$2a$' . hash('whirlpool', $foo); // 128 characters, will be truncated
$hash = crypt($foo, $salt); 
// $hash = $2a$07$b1b2ee48991281a439da2OHi1vZF8Z2zIA.8njYZKR.9iBehxLoIC

I've noticed I can provide some variance in the characters which are here shown as '07$', for example 04$ and 15$ both work, but 01$ through 03$ don't work (generates a blank string), and values such as 99$ and 85$ cause it to revert to STD_DES again.

The Question:

What is the significance of those three characters following the '$2a$' string which, as I am lead to believe by the manual, instruct the crypt function to use the blowfish method.

According to the manual, '$2a$' should be enough to instruct crypt() to use the blowfish method; what, then, is the significance of the following three characters? What then, is the correct format for a salt, if these three characters are so significant?

Answer 1

The number following the 2a specifies the log2 of the number of rounds to perform. For example, 10 means do 1024 rounds. Usually, 10 is normal. Don't use numbers that are too big, or your password will take forever to verify.

See Why does BCrypt.net GenerateSalt(31) return straight away? for something related. :-)