PHP Reference variable when working with recursion function

Question

I have 2 recursion functions to calculate opening stock and closing stock.

public function getClosingStock($previousStock, $product, $periodType, $lower, $upper)
{
    $diffPeriod = 'diffIn' . Str::plural($periodType);
    $addPeriod = "add{$periodType}";

    $totalStocked = $this->getTotalStockInPeriod($product, $periodType, $lower);
    $totalIssued = $this->getTotalIssueInPeriod($product, $periodType, $lower);

    $currentStock = $previousStock + $totalStocked - $totalIssued;

    if ($upper->$diffPeriod($lower) > 0)
    {
        $currentStock = $this->getClosingStock(
            $currentStock, $product, $periodType, $lower->$addPeriod(), $upper
        );
    }

    return $currentStock;
}

public function getOpeningStock($previousStock, $product, $periodType, $lower, $upper)
{
    $subPeriod = "sub{$periodType}";

    return $this->getClosingStock($previousStock, $product, $periodType, $lower, $upper->$subPeriod());
}

Basically, those 2 functions calculate stock from bounded lower time to bounded upper time. For example, bounded lower is December 16th, 2019 and bounded upper is March 31st, 2020. So they will calculate the opening and closing stock of March.


$lower = Carbon::parse('2019-12-16');
$upper = Carbon::parse('2020-03-31');

echo $lower->toDateString() . '<br>'; // Print out: 2019-12-16
echo $upper->toDateString() . '<br>'; // Print out: 2020-03-31

$opening = $this->getOpeningStock(
    0, $product, 'month', $lower, $upper
);

echo $lower->toDateString() . '<br>'; // Print out: 2020-02-16
echo $upper->toDateString() . '<br>'; // Print out: 2020-02-29

$closing = $this->getClosingStock(
    0, $product, 'month', $lower, $upper
);

The problem is after I run the getOpeningStock function my 2 variable $lower and upper were changed, they don't keep the origin value that I set at the beginning. You can see the difference in the comment in my above code. What is the reason cause the difference?

Thank you!

You can declare $lower and $upper both of them as a constant variable, so that your value does not get changed/updated in recursion function. — Sarvil Ajwaliya
– Sarvil Ajwaliya, Commented Mar 28, 2020 at 5:25

kmoser · Accepted Answer · 2020-03-28 05:30:14Z

Clone $lower and $upper, and call $addPeriod() and $subPeriod() on the cloned copies, which you then pass recursively:

public function getClosingStock($previousStock, $product, $periodType, $lower, $upper)
{
    $diffPeriod = 'diffIn' . Str::plural($periodType);
    $addPeriod = "add{$periodType}";

    $totalStocked = $this->getTotalStockInPeriod($product, $periodType, $lower);
    $totalIssued = $this->getTotalIssueInPeriod($product, $periodType, $lower);

    $currentStock = $previousStock + $totalStocked - $totalIssued;

    if ($upper->$diffPeriod($lower) > 0)
    {
    $cloned_lower = clone $lower;
    $currentStock = $this->getClosingStock(
        $currentStock, $product, $periodType, $cloned_lower->$addPeriod(), $upper
    );
    }

    return $currentStock;
}

public function getOpeningStock($previousStock, $product, $periodType, $lower, $upper)
{
    $subPeriod = "sub{$periodType}";

    $cloned_upper = clone $upper;
    return $this->getClosingStock($previousStock, $product, $periodType, $lower, $cloned_upper->$subPeriod());
}

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