Method toBin

/**
 * toBin
 *
 * Accepts a base32 string and returns an ascii binary string
 *
 * @param   string  $str  The base32 string to convert
 *
 * @return  string  Ascii binary string
 *
 * @throws  \Exception
 */
private function toBin($str)
{
    if (!preg_match('/^[' . self::CSRFC3548 . ']+$/', $str)) {
        throw new \Exception('Must match character set');
    }
    // Convert the base32 string back to a binary string
    $str = implode('', array_map(array($this, '_mapbin'), str_split($str)));
    // Remove the extra 0's we added
    $str = preg_replace('/000(.{5})/', '$1', $str);
    // Unpad if necessary
    $length = \strlen($str);
    $rbits = $length & 7;
    if ($rbits > 0) {
        $str = substr($str, 0, $length - $rbits);
    }
    return $str;
}