// SET THE CARD NUMBER TO BE TESTED
$card_number = '';



// TEST THE CARD NUMBER
if (is_card_fake($card_number)) {
	print "It's Possible This Card Is Valid";
}
else {
	print "Card Cannot Possibly Be Valid.";
}



function is_card_fake($card_number) {



	$number_sum = 0;




	// SPLIT THE CARD NUMBER INTO 16 INDIVIDUAL DIGITS
	$digits = preg_split('//', $card_number, -1, PREG_SPLIT_NO_EMPTY);




	// LOOP THROUGH THE DIGITS
	for ($i = 0; $i < strlen($card_number); $i++) {



		// CHECK TO SEE IF WE SHOULD DOUBLE THE NUMBER OR JUST ADD IT
		if ($i % 2 == 0) {



			// DOUBLE THE DIGITS
			$double_number = $digits[$i] * 2;




			// IF THE DOUBLED NUMBER IS 10 OR MORE, SPLIT IT APART AND ADD EACH PIECE TO THE SUM
			if ($double_number > 9) {
				$doubled_digits = preg_split('//', $double_number, -1, PREG_SPLIT_NO_EMPTY);
				$number_sum += $doubled_digits[0] + $doubled_digits[1];
			}
			// OTHERWISE, JUST ADD THE DOUBLED NUMBER TO THE SUM
			else {
				$number_sum += $double_number;
			}



		}
		// WE DO NOT HAVE TO DOUBLE THESE DIGITS; JUST ADD THEM TO THE SUM
		else {
			$number_sum += $digits[$i];
		}


	
	}





	// CHECK TO SEE IF THE SUM OF THE DIGITS CAN BE DIVIDED BY 10
	if ($number_sum % 10 == 0) {
		return true;
	}
	else {
		return false;
	}



}

This function checks to see if a credit card number cannot possibly be valid.  Note: It does NOT check if a card is valid.  A card number can pass this check and still be invalid, but this script will identify card numbers that cannot possibly be valid.

Fake Credit Card Checker

PHP