Função para cálculo de juros baseado no calc.js

calculo.php

<?php

function calculo_taxa_juros($numeroParcelas, $valorParcelas, $valorTotal, $fv = null, $tipo = null, $guess = null) {    
    print('$numeroParcelas: ' . $numeroParcelas) . PHP_EOL;
    print('$valorParcelas: ' . $valorParcelas) . PHP_EOL;
    print('$valorTotal: ' . $valorTotal) . PHP_EOL;
    print('$fv: ' . $fv) . PHP_EOL;
    print('$tipo: ' . $tipo) . PHP_EOL;
    print('$guess: ' . $guess) . PHP_EOL;
    
	//function comp_ir(np,ir,$valorParcelas,$valorTotal,$fv,pb)
    if ($guess == null) $guess = 0.01;
	  if ($fv == null) $fv = 0;
	  if ($tipo == null) $tipo = 0;
	  
      //FROM MS http://office.microsoft.com/en-us/excel-help/rate-HP005209232.aspx
      define('FINANCIAL_MAX_ITERATIONS', 128);//Bet accuracy with 128
      define('FINANCIAL_PRECISION', 0.0000001);//1.0e-8

      $y = null;
      $y0 = null; 
      $y1 = null;
      $x0 = null; 
      $x1 = 0;
      $f = 0;
      $i = 0;
      
      $rate = $guess;
      if (abs($rate) < FINANCIAL_PRECISION) {
         $y = $valorTotal * (1 + $numeroParcelas * $rate) + $valorParcelas * (1 + $rate * $tipo) * $numeroParcelas + $fv;
      } else {
         $f = exp($numeroParcelas * log(1 + $rate));
         $y = $valorTotal * $f + $valorParcelas * (1 / $rate + $tipo) * ($f - 1) + $fv;
      }
      $y0 = $valorTotal + $valorParcelas * $numeroParcelas + $fv;
      $y1 = $valorTotal * $f + $valorParcelas * (1 / $rate + $tipo) * ($f - 1) + $fv;

      // find root by Newton secant method
      $i = $x0 = 0.0;
      $x1 = $rate;
      while ((abs($y0 - $y1) > FINANCIAL_PRECISION) && ($i < FINANCIAL_MAX_ITERATIONS)) {
         $rate = ($y1 * $x0 - $y0 * $x1) / ($y1 - $y0);
         $x0 = $x1;
         $x1 = $rate;

         if (abs($rate) < FINANCIAL_PRECISION) {
            $y = $valorTotal * (1 + $numeroParcelas * $rate) + $valorParcelas * (1 + $rate * $tipo) * $numeroParcelas + $fv;
         } else {
            $f = exp($numeroParcelas * log(1 + $rate));
            $y = $valorTotal * $f + $valorParcelas * (1 / $rate + $tipo) * ($f - 1) + $fv;
         }

         $y0 = $y1;
         $y1 = $y;
         ++$i;
      }
      return round($rate*100, 2);
}

echo comp_ir(50, 787, -30000);