Php debugging fun or 13 != 13

Alerted by a recent report about a bug in the MageBase DPS extension I went on a debugging tour. In the particular user setup the extension errorred out in this line

            if ((float)$resultXml->AmountSettlement != $order->getBaseGrandTotal()) {
                         throw new Exception("Mismatched totals");
            }

even though it shouldn’t have.

Sending both values to the Magento log directly before this line revealed the following:
(float)$resultXml->AmountSettlement = 13;
$order->getBaseGrandTotal() = 13;

Clearly something is going on since to the untrained eye 13 = 13 seems like the same amounts.

Thinking that maybe the comparison also takes type into consideration we tried by adding an additional (float) cast in there, $order->getBaseGrandTotal() is after all saved in the database as a mysql decimal field “13.0000″

            if ((float)$resultXml->AmountSettlement != (float)$order->getBaseGrandTotal()) {
                         throw new Exception("Mismatched totals");
            }

Unfortunately the outcome was the same.

The solution we have come up with is to make the comparison a bit softer:

            if (abs((float)$resultXml->AmountSettlement - $order->getBaseGrandTotal() >0.0005)) {

If the two absolute values of the two amounts are within 5% of a cent it is good enough for me.

Have you come across a similar behaviour with php before? Any other explanations you know of?

ps: The change will be uploaded to Magento Connect with the next version.

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