Here's a better version of the getDigestNotation() function I posted earlier. (The first version had a bug in the argument checking.)

<?php
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;
        $minCharLength = 2;

        while (strlen($chars) >= ($minCharLength *= 2)) {
            $bitsPerCharacter++;
        }

        unset($minCharLength);
    }

    $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;
}
?>