Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Financial.php
public static function ISPMT(...$args)
    {
        // Return value
        $returnValue = 0;

        // Get the parameters
        $aArgs = Functions::flattenArray($args);
        $interestRate = array_shift($aArgs);
        $period = array_shift($aArgs);
        $numberPeriods = array_shift($aArgs);
        $principleRemaining = array_shift($aArgs);

        // Calculate
        $principlePayment = ($principleRemaining * 1.0) / ($numberPeriods * 1.0);
        for ($i = 0; $i <= $period; ++$i) {
            $returnValue = $interestRate * $principleRemaining * -1;
            $principleRemaining -= $principlePayment;
            // principle needs to be 0 after the last payment, don't let floating point screw it up
            if ($i == $numberPeriods) {
                $returnValue = 0;
            }
        }

        return $returnValue;
    }
public static function ISPMT(...$args)
    {
        // Return value
        $returnValue = 0;

        // Get the parameters
        $aArgs = Functions::flattenArray($args);
        $interestRate = array_shift($aArgs);
        $period = array_shift($aArgs);
        $numberPeriods = array_shift($aArgs);
        $principleRemaining = array_shift($aArgs);

        // Calculate
        $principlePayment = ($principleRemaining * 1.0) / ($numberPeriods * 1.0);
        for ($i = 0; $i <= $period; ++$i) {
            $returnValue = $interestRate * $principleRemaining * -1;
            $principleRemaining -= $principlePayment;
            // principle needs to be 0 after the last payment, don't let floating point screw it up
            if ($i == $numberPeriods) {
                $returnValue = 0;
            }
        }

        return $returnValue;
    }
ISPMT.

Returns the interest payment for an investment based on an interest rate and a constant payment schedule.

Excel Function:
=ISPMT(interest_rate, period, number_payments, PV)

interest_rate is the interest rate for the investment

period is the period to calculate the interest rate.  It must be betweeen 1 and number_payments.

number_payments is the number of payments for the annuity

PV is the loan amount or present value of the payments