Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Financial.php
public static function EFFECT($nominal_rate = 0, $npery = 0)
    {
        $nominal_rate = Functions::flattenSingleValue($nominal_rate);
        $npery = (int) Functions::flattenSingleValue($npery);

        // Validate parameters
        if ($nominal_rate <= 0 || $npery < 1) {
            return Functions::NAN();
        }

        return pow((1 + $nominal_rate / $npery), $npery) - 1;
    }
public static function EFFECT($nominal_rate = 0, $npery = 0)
    {
        $nominal_rate = Functions::flattenSingleValue($nominal_rate);
        $npery = (int) Functions::flattenSingleValue($npery);

        // Validate parameters
        if ($nominal_rate <= 0 || $npery < 1) {
            return Functions::NAN();
        }

        return pow((1 + $nominal_rate / $npery), $npery) - 1;
    }
EFFECT.

Returns the effective interest rate given the nominal rate and the number of
compounding payments per year.

Excel Function:
EFFECT(nominal_rate,npery)

@category Financial Functions

@param float $nominal_rate Nominal interest rate
@param int $npery Number of compounding payments per year

@return float