Sometimes you want the digest in both readable notation (such as hexadecimal) and raw binary. At other times you want the digest in a notation other than hexadecimal.

The following getDigestNotation() function takes a binary string and returns it in base 2, 4, 8, 16, 32, or 64 notation. It works with sha1(), md5(), hash(), or anything else that can output a raw binary string.

It works similar to the session.hash_bits_per_character php.ini configuration option.

You can specify which characters to use for each position, or use the default, which matches session.hash_bits_per_character (0-9, a-z, A-Z, "-", ","). The practical range of bits to use per character ($bitsPerCharacter) is 1 to 6; you may use more, but you will have to provide your own base character string ($chars) that is at least pow(2, $bitsPerCharacter) characters long. So even with 7 bits per character you need to specify a value for $chars that is 128 characters long, which exceeds the number of printable ASCII characters.

The output's radix relates to the value of $bitsPerCharacter as follows:
1: base-2 (binary)
2: base-4
3: base-8 (octal)
4: base-16 (hexadecimal)
5: base-32
6: base-64

<?php
$raw = sha1(uniqid(mt_rand(), TRUE), TRUE);

echo getDigestNotation($raw, 6);

function getDigestNotation($rawDigest, $bitsPerCharacter, $chars = NULL)
{
    if ($chars === NULL || strlen($chars) < 2) {
        $chars '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-,';
    }

    if ($bitsPerCharacter < 1) {
        // $bitsPerCharacter must be at least 1
        $bitsPerCharacter = 1;

    } elseif (strlen($chars) < pow(2, $bitsPerCharacter)) {
        // Character length of $chars is too small for $bitsPerCharacter
        // Set $bitsPerCharacter to greatest value allowed by length of $chars
        $bitsPerCharacter = 1;

        do {
            $bitsPerCharacter++;
        } while (strlen($chars) > pow(2, $bitsPerCharacter));
    }

    $bytes = unpack('C*', $rawDigest);
    $byteCount = count($bytes);

    $out = '';
    $byte = array_shift($bytes);
    $bitsRead = 0;

    for ($i = 0; $i < $byteCount * 8 / $bitsPerCharacter; $i++) {

        if ($bitsRead + $bitsPerCharacter > 8) {
            // Not enough bits remain in this byte for the current character
            // Get remaining bits and get next byte
            $oldBits = $byte - ($byte >> 8 - $bitsRead << 8 - $bitsRead);

            if (count($bytes) == 0) {
                // Last bits; match final character and exit loop
                $out .= $chars[$oldBits];
                break;
            }

            $oldBitCount = 8 - $bitsRead;
            $byte = array_shift($bytes);
            $bitsRead = 0;

        } else {
            $oldBitCount = 0;
        }

        // Read only the needed bits from this byte
        $bits = $byte >> 8 - ($bitsRead + ($bitsPerCharacter - $oldBitCount));
        $bits = $bits - ($bits >> $bitsPerCharacter - $oldBitCount << $bitsPerCharacter - $oldBitCount);
        $bitsRead += $bitsPerCharacter - $oldBitCount;

        if ($oldBitCount > 0) {
            // Bits come from seperate bytes, add $oldBits to $bits
            $bits = ($oldBits << $bitsPerCharacter - $oldBitCount) | $bits;
        }

        $out .= $chars[$bits];
    }

    return $out;
}
?>

Lastly, depending on the digest length, there may be fewer bits remaining for the last character than $bitsPerCharacter, so the last character will be smaller. The same thing happens with PHP's session ID generator, when 5 or 6 is used for session.hash_bits_per_character.
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