PHP comparison issues with MySQL data

[reakt]

Hi all. I am having an issue with PHP comparing variables and returning inconsistent (and i think incorrect) results. Here is my script:

function isItMeOr($invoiceId) {

// Tally invoice amount
$invTotal = 0;
$sql8 = mysql_query("SELECT qty, discountPercent, amount
	FROM Invoice_Items
	WHERE Invoice_Items.invoiceId = '$invoiceId'
	AND Invoice_Items.active = '1'");
while ($result8 = mysql_fetch_array($sql8)) {
	$qty = $result8['qty'];
	echo "qty: $qty\n";
	$amount = $result8['amount'];
	echo "amount: $amount\n";
	$discountPercent = $result8['discountPercent'];
	echo "discountPercent: $discountPercent\n";
	$invTotal = $invTotal + ($qty * round(($amount - ($amount * ($discountPercent / 100))), 2));
	echo "invTotal: $invTotal\n";
}
echo "\n";

// Tally payments
$invPayments = 0;
$sql9 = mysql_query("SELECT amount FROM Payment_Allocations WHERE invoiceId = '$invoiceId'");
while ($result9 = mysql_fetch_array($sql9)) {
	$payAmount = $result9['amount'];
	echo "payAmount: $payAmount\n";
	$invPayments = $invPayments + $payAmount;
	echo "invPayments: $invPayments\n";
}
echo "\n";

// Compare
echo "invTotal: $invTotal (" . gettype($invTotal) . ")\n";
echo "invPayments: $invPayments (" . gettype($invPayments) . ")\n";
if ($invTotal == $invPayments) {
	echo "invTotal == invPayments\n";
}
if ($invTotal < $invPayments) {
	echo "invTotal < invPayments\n";
}
if ($invTotal > $invPayments) {
	echo "invTotal > invPayments\n";
}

// Convert to string and compare again
$invTotalAsStr = (string) $invTotal;
$invPaymentsAsStr = (string) $invPayments;
echo "invTotalAsStr: $invTotalAsStr (" . gettype($invTotalAsStr) . ")\n";
echo "invPaymentsAsStr: $invPaymentsAsStr (" . gettype($invPaymentsAsStr) . ")\n";
if ($invTotalAsStr == $invPaymentsAsStr) {
	echo "invTotalAsStr == invPaymentsAsStr\n";
}
if ($invTotalAsStr < $invPaymentsAsStr) {
	echo "invTotalAsStr < invPaymentsAsStr\n";
}
if ($invTotalAsStr > $invPaymentsAsStr) {
	echo "invTotalAsStr > invPaymentsAsStr\n";
}

echo "\n\n";
}

isItMeOr('15126');
isItMeOr('15171');

 

And here is the output:

 

qty: 1
amount: 55
discountPercent: 0
invTotal: 55
qty: 1
amount: 49.95
discountPercent: 0
invTotal: 104.95

payAmount: 104.95
invPayments: 104.95

invTotal: 104.95 (double)
invPayments: 104.95 (double)
invTotal == invPayments
invTotalAsStr: 104.95 (string)
invPaymentsAsStr: 104.95 (string)
invTotalAsStr == invPaymentsAsStr


qty: 1
amount: 64.9
discountPercent: 0
invTotal: 64.9
qty: 1
amount: 49.95
discountPercent: 0
invTotal: 114.85
qty: 1
amount: 99
discountPercent: 0
invTotal: 213.85

payAmount: 213.85
invPayments: 213.85

invTotal: 213.85 (double)
invPayments: 213.85 (double)
invTotal > invPayments
invTotalAsStr: 213.85 (string)
invPaymentsAsStr: 213.85 (string)
invTotalAsStr == invPaymentsAsStr

 

As you can see, the results for invoice '15126' come back as you would expect. The result for '15171' says invTotal > invPayments even though they are the same. Changing them to strings fixes the issue, however I'd still like to know why it's happening in the first place. Does anyone know what's going on here or what I can do to avoid such a pitfall in future? Thanks

[reakt]

Boss just linked me to this http://php.net/manual/en/language.types.float.php

 

The php manual actually says "never compare floating point numbers for equality". Guess I'll know for next time.