=.='' what wrong ? just simple code, however error.

<?
$credithold = 100;
for($i=1;$i<=1000;$i++){
$credithold -= 0.1;
echo "$credithold<br />";
}
//i don't know why, when run this code, on 91.3 after expect is 91.2,
however......91.200000000001
//who can help me ? and tell me why ?

//Thank You.
?>


.