CompCalc

<?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	// [period, principal value, amount withdrawn so far, total return so far, total ROI]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 Then substract the withdrawn amount. 
			// This amount is: Withdrawn = Profit X (compounding percentage / 100)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;
	}
}
?>