public function ActualAPR($amount, $closingCost, $interest, $length, $periodPayment)
{
if ($amount <= 0) {
return 0;
}
if ($closingCost == $amount) {
return 0;
}
if ($closingCost == 0) {
return $interest;
}
$totalAmount = $amount - $closingCost;
$aprA = $interest;
$aprB = $interest;
$aprX = 0;
do {
$aprA = $aprA / 2;
$payment = Calc::PeriodPayment($totalAmount, $aprA, $length);
} while ($payment > $periodPayment);
do {
$aprB = $aprB * 2;
$payment = Calc::PeriodPayment($totalAmount, $aprB, $length);
} while ($payment < $periodPayment);
do {
$aprX = ($aprA + $aprB) / 2;
$payment = Calc::PeriodPayment($totalAmount, $aprX, $length);
if ($payment > $periodPayment) {
$aprB = $aprX;
} else {
$aprA = $aprX;
}
} while (abs($payment - $periodPayment) > 0.001);
return $aprX;
}