rounding - PHP: precision in mathematical operations

i've a question on PHP and mathematical operations.

why 9000.05 - 9000 return 0.049999999999272 and not 0.05 ???

please look that example

function calculate ( $financed, $months, $rate){
    $installment = 782.90;  
    $rate= $rate / 100;

    $delta = 0;

    $OUT = $financed;
    $IN  = 0;

    for ($t = 1; $t <= $months; $t++){
        echo "installment $t \n";
        $installment_act =  $installment / pow((1 + ($rate / 12)), $t);
        $installment_act = round($installment_act, 2);
        echo "installment actualized $installment_act \n\n";

        $IN += $installment_act;
        echo "\t\t\t\t $IN\n";
    }
    echo "\n IN ($IN) - OUT ($OUT) = ";
    $delta = $IN - $OUT;
    echo "delta = $delta\n  ";
}

calculate(9000, 12, 8);


//RESULT

installment 1 
installment actualized 777.72 

                 777.72
installment 2 
installment actualized 772.56 

                 1550.28

installment 3 
installment actualized 767.45 

                 2317.73
installment 4 
installment actualized 762.37 

                 3080.1
installment 5 
installment actualized 757.32 


                 3837.42
installment 6 
installment actualized 752.3 

                 4589.72
installment 7 
installment actualized 747.32 

                 5337.04

installment 8 
installment actualized 742.37 

                 6079.41
installment 9 
installment actualized 737.45 

                 6816.86
installment 10 
installment actualized 732.57 


                 7549.43
installment 11 
installment actualized 727.72 

                 8277.15
installment 12 
installment actualized 722.9 

                 9000.05


IN (9000.05) - OUT (9000) = delta = 0.049999999999272

as you see $IN - $OUT return a wrong result, why? and how could i fix it?

thanks advance for any suggestion
Michele