Xampp Install - Large numbers calcs not working like preceding smaller ones?

//I am calculating payroll contributions to 401k to give employees a baseline outlook for 20 years to check against their portfolio quarterly reports. It all works until numbers start getting larger. Nothing //ginormous...this is the reason I am at a loss. Is there a setting in php.ini that would be bombing?

Sorry...the code formatting tool is acting dumb.

    $Pay_earnings   = $_REQUEST['Pay_earnings'];
    $Perc_contr     = $_REQUEST['Perc_contr']/100;
    $Perc_emp_contr = $_REQUEST['Perc_emp_contr']/100;
    if($Perc_contr > .08){
        $perc_non_matched = $Perc_contr - .08;
        $Perc_contr = .08;
    }else{
        $non_matched_contri = 0;
    }
    $yourContrib    = number_format(round( (($Pay_earnings*$Perc_contr)+($Pay_earnings*$perc_non_matched)), 2));
    $empContrib     = number_format(round(($Pay_earnings*$Perc_contr)*$Perc_emp_contr,2));
    $totalContrib   = number_format($yourContrib+$empContrib);

This works find until my $Perc_contr rises above 40. Then my $totalContrib toward the bottom does not reflect my $yourContrib in the addition. But the actual TD holding the $yourContrib is fine. I am frikin dumbfounded....