function entropy($string)
{
    $frequency = array(
        "а" => 0,
        "б" => 0,
        "в" => 0,
        "г" => 0,
        "д" => 0,
        "е" => 0,
        "ё" => 0,
        "ж" => 0,
        "з" => 0,
        "и" => 0,
        "й" => 0,
        "к" => 0,
        "л" => 0,
        "м" => 0,
        "н" => 0,
        "о" => 0,
        "п" => 0,
        "р" => 0,
        "с" => 0,
        "т" => 0,
        "у" => 0,
        "ф" => 0,
        "х" => 0,
        "ц" => 0,
        "ч" => 0,
        "ш" => 0,
        "щ" => 0,
        "ъ" => 0,
        "ы" => 0,
        "ь" => 0,
        "э" => 0,
        "ю" => 0,
        "я" => 0,
        "a" => 0,
        "b" => 0,
        "c" => 0,
        "d" => 0,
        "e" => 0,
        "f" => 0,
        "g" => 0,
        "h" => 0,
        "i" => 0,
        "j" => 0,
        "k" => 0,
        "l" => 0,
        "m" => 0,
        "n" => 0,
        "o" => 0,
        "p" => 0,
        "q" => 0,
        "r" => 0,
        "s" => 0,
        "t" => 0,
        "u" => 0,
        "v" => 0,
        "w" => 0,
        "x" => 0,
        "y" => 0,
        "z" => 0,
        "0" => 0,
        "1" => 0,
        "2" => 0,
        "3" => 0,
        "4" => 0,
        "5" => 0,
        "6" => 0,
        "7" => 0,
        "8" => 0,
        "9" => 0,
        "." => 0,
        "," => 0,
        ":" => 0,
        ";" => 0,
        "!" => 0,
        "?" => 0,
        " " => 0,
        """ => 0,
        """ => 0,
        "+" => 0,
        "-" => 0,
        "*" => 0,
        "/" => 0,
        "=" => 0,
        "%" => 0,
        "~" => 0,
        "@" => 0,
        "№" => 0,
        "#" => 0,
        "" => 0,
        "|" => 0,
        "_" => 0,
        "&" => 0,
        "^" => 0,
        "$" => 0,
        "`" => 0,
        "(" => 0,
        ")" => 0,
        "[" => 0,
        "]" => 0,
        "{" => 0,
        "}" => 0
    );
    $string = mb_strtolower($string);
    $stringLength = mb_strlen($string);
    $entropy = 0;
    foreach ($frequency as $key => $val)
    {
        $frequency[$key] = substr_count($string, $key);
        if ($frequency[$key] > 0)
        {
            $p = $frequency[$key] / $stringLength;
            $logP = log((1 / $p), 2);
            $H = $p * $logP;
            $entropy = $entropy + $H;
        }
    }
    return round($entropy, 2);
}