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);
}