IRR formula for PHP

gistfile1.txt

<?php

class IRRHelper{

	//Adapted from Javascript version here: https://gist.github.com/ghalimi/4591338
	//
	// Copyright (c) 2012 Sutoiku, Inc. (MIT License)
 	//
	// Some algorithms have been ported from Apache OpenOffice:
	//	 
	/**************************************************************
	 * 
	 * Licensed to the Apache Software Foundation (ASF) under one
	 * or more contributor license agreements.  See the NOTICE file
	 * distributed with this work for additional information
	 * regarding copyright ownership.  The ASF licenses this file
	 * to you under the Apache License, Version 2.0 (the
	 * "License"); you may not use this file except in compliance
	 * with the License.  You may obtain a copy of the License at
	 * 
	 *   http://www.apache.org/licenses/LICENSE-2.0
	 * 
	 * Unless required by applicable law or agreed to in writing,
	 * software distributed under the License is distributed on an
	 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	 * KIND, either express or implied.  See the License for the
	 * specific language governing permissions and limitations
	 * under the License.
	 * 
	 *************************************************************/

	public static function IRR($values, $guess=0.1) {
		// Credits: algorithm inspired by Apache OpenOffice

		// Initialize dates and check that values contains at least one positive value and one negative value
		$dates = array();
		$positive = false;
		$negative = false;
		foreach($values as $index=>$value){
			$dates[] = ($index===0) ? 0 : $dates[$index-1] + 365;
			if($values[$index] > 0) $positive = true;
			if($values[$index] < 0) $negative = true;
		}

		// Return error if values does not contain at least one positive value and one negative value
		if(!$positive || !$negative) return null;

		// Initialize guess and resultRate
		$resultRate = $guess;

		// Set maximum epsilon for end of iteration
		$epsMax = 0.0000000001;

		// Set maximum number of iterations
		$iterMax = 50;

		// Implement Newton's method
		$newRate;
		$epsRate;
		$resultValue;
		$iteration = 0;
		$contLoop = true;
		while($contLoop && (++$iteration < $iterMax)){
			$resultValue = self::irrResult($values, $dates, $resultRate);
			$newRate = $resultRate - $resultValue / self::irrResultDeriv($values, $dates, $resultRate);
			$epsRate = abs($newRate - $resultRate);
			$resultRate = $newRate;
			$contLoop = ($epsRate > $epsMax) && (abs($resultValue) > $epsMax);
		}

		if($contLoop) return null;

		// Return internal rate of return
		return $resultRate;
	}

	// Calculates the resulting amount
	public static function irrResult($values, $dates, $rate){
		$r = $rate + 1;
		$result = $values[0];
		for($i=1;$i<count($values);$i++){
			$result += $values[$i] / pow($r, ($dates[$i] - $dates[0]) / 365);
		}
		return $result;
	}

	// Calculates the first derivation
	public static function irrResultDeriv($values, $dates, $rate){
		$r = $rate + 1;
		$result = 0;
		for($i=1;$i<count($values);$i++){
			$frac = ($dates[$i] - $dates[0]) / 365;
			$result -= $frac * $values[$i] / pow($r, $frac + 1);
		}
		return $result;
	}
	
}

echo IRRHelper::IRR(array(-100,0,120));

i have same problem too, irrResultDeriv return 0 if only 2 value in array

Same problem (division by zero). If anyone has gotten it right, please share (even using some other function).