This class calculates compounded interest on investment, saving or loan.
It allows variable compounding (reinvesting) percentage. Here's a page where you can see the class used: http://sharkinvestor.com/investment-calculator/

<?php
/* This class calculates compounded interest on investment, saving or loan. 
 It allows variable compounding (reinvesting) percentage 
 To do: add different types of loan calculations. 
 Will be useful for all kind of financial calculators and apps. 
 Here's a page where you can see the class used: http://sharkinvestor.com/investment-calculator/ */
class CompCalc
{
    // calculates compounded investment
    // $num_periods - required, number of compounding periods (usually years or months)
    // $investment - required, the initial investment amount
    // $interest - required, interest in % for each period
    // $reinvest_percentage - optional, now much from the return is reinvested
    // $addition_per_period - optional, additional investment made in each period
    // @returns array of arrays - one array for each period:
    // [period, principal value, amount withdrawn so far, total return so far, total ROI]
    // the results should be read as "at the end of period X"
    function inv_calculate($num_periods, $investment, $interest, 
        $reinvest_percentage=100, $addition_per_period=0)
    {
        // validate
        if($num_periods<=0) throw new Exception("Periods should be at least 1");
        if($investment<=0) throw new Exception("Investment amount should be bigger than 0");
        if(!is_numeric($interest)) throw new Exception("Interest should be a number");
        if(!is_numeric($reinvest_percentage)) throw new Exception("Reinvest % should be a number");
        if(!is_numeric($addition_per_period)) throw new Exception("Addition should be a number");

        // convert interest and reinvest_percentage to decimal numbers
        $roi=$interest/100; 
        $cp=$reinvest_percentage/100;

        // Start the calculations

        // STEP 1 & 2:
        // 1. When the user input specific number of periods, you just go thru them
        // 2. for each period calculate: Profit = Principal (Invested amount) X (ROI / 100)     
        $total_withdrawn=0;
        $results=array();       

        for($i = 1 ; $i < ($num_periods+1) ; $i++ )
        {
            $new_principal=!empty($new_principal)?$new_principal:$investment;
            $new_principal=round($new_principal,2);

            $profit = $new_principal * $roi;        
            $profit=round($profit,2);

            // STEP 3:
            // 3. Then substract the withdrawn amount. 
            // This amount is: Withdrawn = Profit X (compounding percentage / 100)
            $addition = $profit * $cp;
            $withdraw=round(($profit-$addition),2); 

            // totoal return
            $total_withdrawn+=$withdraw;

            // total ROI
            $total_roi=($total_withdrawn/$investment)*100;
            $total_roi=round($total_roi);

            $new_principal=$new_principal+$addition;

            // period adddition
            $investment+=$addition_per_period;
            $new_principal+=$addition_per_period;

            $result=array("period"=>$i, "investment_value"=>$new_principal, 
                "return_for_the_period"=>$withdraw, "total_return"=>$total_withdrawn,
                "total_roi"=>$total_roi);
            $results[]=$result; 
        }

        return $results;
    }
}
?>