<?php

/**
 * This method logically validates Turkish VAT number
 *
 * @param string $taxNumber
 * @return bool
 */
public function validateTaxNumber(string $taxNumber): bool
{
    if (strlen($taxNumber) !== 10) {
        return false;
    }

    $total = 0;
    $checkNum = null;

    for ($i = 0; $i < 9; $i++) {
        $tmp1 = ($taxNumber[$i] + (9 - $i)) % 10;
        $tmp2 = ($tmp1 * (2 ** (9 - $i))) % 9;

        if ($tmp1 !== 0 && $tmp2 === 0) {
            $tmp2 = 9;
        }

        $total += $tmp2;
    }

    if ($total % 10 === 0) {
        $checkNum = 0;
    } else {
        $checkNum = 10 - ($total % 10);
    }

    if ((int)$taxNumber[9] !== $checkNum) {
        return false;
    }

    return true;
}