A PHP Function that validates Brazil's CNPJ

validateCNPJ.php

<?php

function validateCNPJ($cnpj)
{
	$parsedCNPJ = preg_replace("/[.\/-]/", "", $cnpj);

	if (!preg_match("/^\d{14}$/", $parsedCNPJ)) {
		return false;
	}

	$weights = [5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2];

	$digits = [0, 0];

	for ($i = 12; $i < 14; $i++) {
		if ($i === 13) {
			$weights = [6, ...$weights];
		}

		for ($j = 0; $j < $i; $j++) {
			if ($j < 12) {
				$digits[$i - 12] += intval($parsedCNPJ[$j]) * $weights[$j];
			} else {
				$digits[1] += $digits[0] * $weights[$j];
			}
		}

		$digits[$i - 12] %= 11;

		if ($digits[$i - 12] < 2) {
			$digits[$i - 12] = 0;
		} else {
			$digits[$i - 12] = 11 - $digits[$i - 12];
		}
	}

	$firstDigitIsValid = $digits[0] === intval($parsedCNPJ[12]);
	$secondDigitIsValid = $digits[1] === intval($parsedCNPJ[13]);

	return $firstDigitIsValid && $secondDigitIsValid;
}

// Random CNPJ number generated at https://www.4devs.com.br/gerador_de_cnpj
$randomCorrectCNPJ = "37.741.477/0001-38";
$randomBrokeCNPJ = "37.741.477/0001-37";

// Will echo 1
echo validateCNPJ($randomCorrectCNPJ);

// Will echo 0
echo validateCNPJ($randomBrokeCNPJ);