Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Statistical.php
public static function COUNTBLANK(...$args)
    {
        $returnValue = 0;

        // Loop through arguments
        $aArgs = Functions::flattenArray($args);
        foreach ($aArgs as $arg) {
            // Is it a blank cell?
            if (($arg === null) || ((is_string($arg)) && ($arg == ''))) {
                ++$returnValue;
            }
        }

        return $returnValue;
    }
public static function COUNTBLANK(...$args)
    {
        $returnValue = 0;

        // Loop through arguments
        $aArgs = Functions::flattenArray($args);
        foreach ($aArgs as $arg) {
            // Is it a blank cell?
            if (($arg === null) || ((is_string($arg)) && ($arg == ''))) {
                ++$returnValue;
            }
        }

        return $returnValue;
    }
COUNTBLANK.

Counts the number of empty cells within the list of arguments

Excel Function:
COUNTBLANK(value1[,value2[, ...]])

@category Statistical Functions

@param mixed ...$args Data values

@return int