Sorry, I through together that sample code really quick. 
Here's a simplified sample code snippet that produces the same results:

$amount = 49.83;
$out=array();
for($i=0; $i<1000; $i++){
	if(!isset($out['test'])) $out['test']=0;
	$out['test'] += $amount;
	echo $out['test'].' + '.$amount.'<br>';
}
echo '<pre>';
print_r($out);
echo '</pre>';
// Messes up at value: 31243.41

Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.

<?	  
echo  $balance.'<br>';
echo  $trbalance.'<br>';
echo $openbalance = $balance - $trbalance.'<br>';	?>
_______________________________________________________________
Output: 
80343.05
38390.74
41952.26
__________________________________________________

So 80343.05 - 38390.74 = 41952.26 (!!!!  should be 41952.31) 

lost 0.05 in simple math.

@junkmail

<?php
$balance = 80343.05;
$trbalance = 38390.74;
echo  $balance.'<br>';
echo  $trbalance.'<br>';
echo $openbalance = $balance - $trbalance.'<br>';

outputs:

80343.05
38390.74
41952.31