HomeLoanTransfer_calculateRate.php

<?php

function calculateRate($nper, $pmt, $pv, $fv, $type, $guess) {
      //FROM MS http://office.microsoft.com/en-us/excel-help/rate-HP005209232.aspx
      $FINANCIAL_MAX_ITERATIONS = 20;//Bet accuracy with 128
      $FINANCIAL_PRECISION = 0.0000001;//1.0e-8

      $y, $y0, $y1, $x0, $x1 = 0, $f = 0, $i = 0;
      $rate = $guess;
      if (abs($rate) < $FINANCIAL_PRECISION) {
         $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
      } else {
         $f = exp($nper * log(1 + $rate));
         $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
      }
      $y0 = $pv + $pmt * $nper + $fv;
      $y1 = $pv * $f + $pmt * (1 / $rate + $type) * ($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 = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
         } else {
            $f = exp($nper * log(1 + $rate));
            $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
         }

         $y0 = $y1;
         $y1 = $y;
         ++$i;
      }
      return $rate;
   }

echo calculateRate();
?>