Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Statistical.php
public static function TRIMMEAN(...$args)
    {
        $aArgs = Functions::flattenArray($args);

        // Calculate
        $percent = array_pop($aArgs);

        if ((is_numeric($percent)) && (!is_string($percent))) {
            if (($percent < 0) || ($percent > 1)) {
                return Functions::NAN();
            }
            $mArgs = [];
            foreach ($aArgs as $arg) {
                // Is it a numeric value?
                if ((is_numeric($arg)) && (!is_string($arg))) {
                    $mArgs[] = $arg;
                }
            }
            $discard = floor(self::COUNT($mArgs) * $percent / 2);
            sort($mArgs);
            for ($i = 0; $i < $discard; ++$i) {
                array_pop($mArgs);
                array_shift($mArgs);
            }

            return self::AVERAGE($mArgs);
        }

        return Functions::VALUE();
    }
public static function TRIMMEAN(...$args)
    {
        $aArgs = Functions::flattenArray($args);

        // Calculate
        $percent = array_pop($aArgs);

        if ((is_numeric($percent)) && (!is_string($percent))) {
            if (($percent < 0) || ($percent > 1)) {
                return Functions::NAN();
            }
            $mArgs = [];
            foreach ($aArgs as $arg) {
                // Is it a numeric value?
                if ((is_numeric($arg)) && (!is_string($arg))) {
                    $mArgs[] = $arg;
                }
            }
            $discard = floor(self::COUNT($mArgs) * $percent / 2);
            sort($mArgs);
            for ($i = 0; $i < $discard; ++$i) {
                array_pop($mArgs);
                array_shift($mArgs);
            }

            return self::AVERAGE($mArgs);
        }

        return Functions::VALUE();
    }
TRIMMEAN.

Returns the mean of the interior of a data set. TRIMMEAN calculates the mean
taken by excluding a percentage of data points from the top and bottom tails
of a data set.

Excel Function:
TRIMEAN(value1[,value2[, ...]], $discard)

@category Statistical Functions

@param mixed $args Data values
@param float $discard Percentage to discard

@return float