Simple math failing - PHP Bug?

["Brian T. Allen" <brian@xxxxxxxxxxxxxxx>]

Hi,

OK, I'm totally stumped by this.   This should be the simplest math 
imaginable (addition and subtraction), but PHP is coming up with the 
wrong answer!

I've checked on 3 different machines (all linux) running both PHP 4 and 
PHP 5.

Here is the code to duplicate the problem:

-----------------------------------
<?php

$a = 2503.54;
$b = 303.55;
$c = 202.13;

$total = 0;
$total += $a;
$total += $b;
$total += $c;

$added_total = $total;

echo "$a + $b + $c = $total<br>";
echo "Actual: $total<br><br>";

$total -= $a;
$total -= $b;
$total -= $c;

echo "$added_total - $a - $b - $c = 0<br>";
echo "Actual: $total<br><br>";

?>
-----------------------------------

This ~should~ output this:

      2503.54 + 303.55 + 202.13 = 3009.22
      Actual: 3009.22

      3009.22 - 2503.54 - 303.55 - 202.13 = 0
      Actual: 0

But instead it outputs this:

      2503.54 + 303.55 + 202.13 = 3009.22
      Actual: 3009.22

      3009.22 - 2503.54 - 303.55 - 202.13 = 0
      Actual: 2.84217094304E-13


Now I'm the first to admit that 0.000000000000284217094304 and 0 are 
really very close, but when you are testing a variable to see if it 
equals 0, it is the difference between true and false.

This seems very, very basic to me, but I can't find the flaw in my code.

Is this legitimately bad math on the part of PHP?

Thanks,
Brian

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