Zend\Diactoros\HeaderSecurity::filter PHP Method

    public static function filter($value)
    {
        $value = (string) $value;
        $length = strlen($value);
        $string = '';
        for ($i = 0; $i < $length; $i += 1) {
            $ascii = ord($value[$i]);
            // Detect continuation sequences
            if ($ascii === 13) {
                $lf = ord($value[$i + 1]);
                $ws = ord($value[$i + 2]);
                if ($lf === 10 && in_array($ws, [9, 32], true)) {
                    $string .= $value[$i] . $value[$i + 1];
                    $i += 1;
                }
                continue;
            }
            // Non-visible, non-whitespace characters
            // 9 === horizontal tab
            // 32-126, 128-254 === visible
            // 127 === DEL
            // 255 === null byte
            if ($ascii < 32 && $ascii !== 9 || $ascii === 127 || $ascii > 254) {
                continue;
            }
            $string .= $value[$i];
        }
        return $string;
    }