This is expected behaviour.  When rounding on exact halves we round down on evens and up on odds.  If you want to always force it in one direction on a .5 (or .05 in your case) add or substract a tiny fuzz factor.  The reason behind rounding half the values down and the other half up is to avoid the classical banking problem where if you always rounded down you would be stealing money from your customers, or if you always rounded up you would end up over time losing money.  By averaging it out through evens and odds you statistically break even.  If you want to learn more, search the web for terms such as "round to even" or "banker's rounding" and you will find that most languages actually do it this way.

hmm, actually this changed between PHP 4.2.2, PHP 4.2.3 an PHP 4.3.0:

[derick@kossu derick]$ cat foo.php   
<?php
$mul = 1;
$one = 5;
$two = 0.9;

echo sprintf("%1.2f", round($mul * round($one * (1 + ($two / 100)), 2), 2));
?>

[derick@kossu derick]$ php-4.2.2 foo.php 
5.04

[derick@kossu derick]$ php-4.2.3 foo.php 
5.05zend_hash.c(98) : Bailed out without a bailout address!

[derick@kossu derick]$ php-4.3.0 foo.php 
5.05

[derick@kossu derick]$ php-4.3.0dev foo.php 
5.05

[derick@kossu derick]$ php-5.0.0dev foo.php 
5.04

So something is definitely screwed in the PHP_4_3 branch

Derick

With this script:

<?php

$mul = 1;
$one = 5;
$two = 0.9;

ini_set("precision", 24);

$res = 5 * (1 + (0.9 / 100));
$bar = round($res, 2);
$foo = round(1 * $bar, 2);

echo $res, "\n", $bar, "\n", $foo, "\n";

echo sprintf("%1.2f", $foo);

?>

Result with PHP 4.3.3-dev:
5.04499999999999992894573
5.04999999999999982236432
5.04999999999999982236432
5.05

Result with PHP 4.2.3:
5.04499999999999992894573
5.04000000000000003552714
5.04000000000000003552714
5.04

Nokia 6120 (the $res):
5.045

TI-32:
5.045

With pen and paper:
5.045

Round 5.045 with precision of 2 == 5.05

So I think PHP 4.3.2 actually works as expected 
and anything before (and after, PHP 5? :) doesn't.

Reclassified as ZE2 bug.

This bug has been fixed in CVS.

In case this was a PHP problem, snapshots of the sources are packaged
every three hours; this change will be in the next snapshot. You can
grab the snapshot at http://snaps.php.net/.
 
In case this was a documentation problem, the fix will show up soon at
http://www.php.net/manual/.

In case this was a PHP.net website problem, the change will show
up on the PHP.net site and on the mirror sites in short time.
 
Thank you for the report, and for helping us make PHP better.