libphonenumber\PhoneNumberUtil::normalizeDigits PHP Method

    public static function normalizeDigits($number, $keepNonDigits)
    {
        $normalizedDigits = "";
        $numberAsArray = preg_split('/(?<!^)(?!$)/u', $number);
        foreach ($numberAsArray as $character) {
            if (is_numeric($character)) {
                $normalizedDigits .= $character;
            } elseif ($keepNonDigits) {
                $normalizedDigits .= $character;
            }
            // If neither of the above are true, we remove this character.
            // Check if we are in the unicode number range
            if (array_key_exists($character, static::$numericCharacters)) {
                $normalizedDigits .= static::$numericCharacters[$character];
            }
        }
        return $normalizedDigits;
    }