libphonenumber\PhoneNumberUtil::getInvalidExampleNumber PHP Method

    public function getInvalidExampleNumber($regionCode)
    {
        if (!$this->isValidRegionCode($regionCode)) {
            return null;
        }
        // We start off with a valid fixed-line number since every country supports this. Alternatively
        // we could start with a different number type, since fixed-line numbers typically have a wide
        // breadth of valid number lengths and we may have to make it very short before we get an
        // invalid number.
        $desc = $this->getNumberDescByType($this->getMetadataForRegion($regionCode), PhoneNumberType::FIXED_LINE);
        if ($desc->getExampleNumber() == '') {
            // This shouldn't happen; we have a test for this.
            return null;
        }
        $exampleNumber = $desc->getExampleNumber();
        // Try and make the number invalid. We do this by changing the length. We try reducing the
        // length of the number, since currently no region has a number that is the same length as
        // MIN_LENGTH_FOR_NSN. This is probably quicker than making the number longer, which is another
        // alternative. We could also use the possible number pattern to extract the possible lengths of
        // the number to make this faster, but this method is only for unit-testing so simplicity is
        // preferred to performance.  We don't want to return a number that can't be parsed, so we check
        // the number is long enough. We try all possible lengths because phone number plans often have
        // overlapping prefixes so the number 123456 might be valid as a fixed-line number, and 12345 as
        // a mobile number. It would be faster to loop in a different order, but we prefer numbers that
        // look closer to real numbers (and it gives us a variety of different lengths for the resulting
        // phone numbers - otherwise they would all be MIN_LENGTH_FOR_NSN digits long.)
        for ($phoneNumberLength = mb_strlen($exampleNumber) - 1; $phoneNumberLength >= static::MIN_LENGTH_FOR_NSN; $phoneNumberLength--) {
            $numberToTry = mb_substr($exampleNumber, 0, $phoneNumberLength);
            try {
                $possiblyValidNumber = $this->parse($numberToTry, $regionCode);
                if (!$this->isValidNumber($possiblyValidNumber)) {
                    return $possiblyValidNumber;
                }
            } catch (NumberParseException $e) {
                // Shouldn't happen: we have already checked the length, we know example numbers have
                // only valid digits, and we know the region code is fine.
            }
        }
        // We have a test to check that this doesn't happen for any of our supported regions.
        return null;
    }