Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Engineering.php
public static function IMSUM(...$complexNumbers)
    {
        // Return value
        $returnValue = self::parseComplex('0');
        $activeSuffix = '';

        // Loop through the arguments
        $aArgs = Functions::flattenArray($complexNumbers);
        foreach ($aArgs as $arg) {
            $parsedComplex = self::parseComplex($arg);

            if ($activeSuffix == '') {
                $activeSuffix = $parsedComplex['suffix'];
            } elseif (($parsedComplex['suffix'] != '') && ($activeSuffix != $parsedComplex['suffix'])) {
                return Functions::NAN();
            }

            $returnValue['real'] += $parsedComplex['real'];
            $returnValue['imaginary'] += $parsedComplex['imaginary'];
        }

        if ($returnValue['imaginary'] == 0.0) {
            $activeSuffix = '';
        }

        return self::COMPLEX($returnValue['real'], $returnValue['imaginary'], $activeSuffix);
    }
public static function IMSUM(...$complexNumbers)
    {
        // Return value
        $returnValue = self::parseComplex('0');
        $activeSuffix = '';

        // Loop through the arguments
        $aArgs = Functions::flattenArray($complexNumbers);
        foreach ($aArgs as $arg) {
            $parsedComplex = self::parseComplex($arg);

            if ($activeSuffix == '') {
                $activeSuffix = $parsedComplex['suffix'];
            } elseif (($parsedComplex['suffix'] != '') && ($activeSuffix != $parsedComplex['suffix'])) {
                return Functions::NAN();
            }

            $returnValue['real'] += $parsedComplex['real'];
            $returnValue['imaginary'] += $parsedComplex['imaginary'];
        }

        if ($returnValue['imaginary'] == 0.0) {
            $activeSuffix = '';
        }

        return self::COMPLEX($returnValue['real'], $returnValue['imaginary'], $activeSuffix);
    }
IMSUM.

Returns the sum of two or more complex numbers in x + yi or x + yj text format.

Excel Function:
IMSUM(complexNumber[,complexNumber[,...]])

@param string ...$complexNumbers Series of complex numbers to add

@return string