BarcodeGenerator::ean_13_normalize

private function ean_13_normalize($data) {
	$data = preg_replace('/[^0-9*]/', '', $data);
	/* Set length to 13 digits. */
	if (strlen($data) < 13) {
		return '0' . $this->upc_a_normalize($data);
	} else if (strlen($data) > 13) {
		$left = substr($data, 0, 7);
		$right = substr($data, -6);
		$data = $left . $right;
	}
	/* Replace * with missing or check digit. */
	while (($o = strrpos($data, '*')) !== false) {
		$checksum = 0;
		for ($i = 0; $i < 13; $i++) {
			$digit = (int)$data[$i];
			$checksum += (($i % 2) ? 3 : 1) * $digit;
		}
		$checksum *= (($o % 2) ? 3 : 9);
		$left = substr($data, 0, $o);
		$center = substr($checksum, -1);
		$right = substr($data, $o + 1);
		$data = $left . $center . $right;
	}
	return $data;
}