There is a mathematical bug in PHP 4.0.6, 4.1.2 and 4.2.1.
When I try to run the following script:
<?php
echo "20.91-20.85=";
eCho 20.91-20.85;
echo "<br>";
echo "10.33-10.23=";
echo 10.33-10.23;
echo "<br>";
echo "20.33-20.23=";
echo 20.33-20.23;
echo "<br>";
echo "21.33-21.23=";
echo 21.33-21.23;
echo "<br>";
echo "30.33-30.23=";
echo 30.33-30.23;
echo "<br>";

I got the following result,

20.91-20.85=0.059999999999999		(Wrong)
10.33-10.23=0.1  					(Right)
20.33-20.23=0.099999999999998		(Wrong)
21.33-21.23=0.099999999999998		(Wrong)
30.33-30.23=0.099999999999998		(Wrong)

Obviously it??s wrong.