Description:
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I've been setting up our system to receive payments via Paypal. Paypal talks via IPN to a URL that I specify. In my code I have logic to verify the transaction. 

On one line I ran into the problem of addition when verifying transaction amounts.

I have tried this test on Win32 running PHP 5.2.3 (Basic AppServ install) and on our live linux environment PHP 5.2.6 with both showing the same results.

Reproduce code:
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$SubTotal = "67.43";
$Tax = "3.37";

$Amount = $SubTotal + $Tax;

"<div>".var_dump($Amount)."</div>";
$Array = array("mc_gross" => "70.80");

echo ($Amount == $Array['mc_gross']) ? $Array['mc_gross'].' is equal to '.$Amount : 'These values are not equal. '.$Array['mc_gross']." is not equal to ".$Amount;

echo '<br /><br />Second Test<br /><br />';
$Amount = 70.8;
"<div>".var_dump($Amount)."</div>";

echo ($Amount == $Array['mc_gross']) ? $Array['mc_gross'].' is equal to '.$Amount : 'These values are not equal. '.$Array['mc_gross']." is not equal to ".$Amount;

Expected result:
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The variable mc_gross, a string with a string value "70.80" should equal the variable $Amount with the float value of 70.8

Actual result:
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The first test adds the two variables ($Subtotal and $Tax) together into the variable $Amount. Using var_dump php tells us that $Amount is a float with the value 70.8

When comparing that variable to the one supplied by Paypal (mc_gross) php says that the values are not equal to each other.

If you redeclare the variable $Amount and with the value 70.8 and try again, php says the values are equal. 

var_dump confirms the variable types and value.