Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Functions.php
public static function isFormula($value = '', Cell $pCell = null)
    {
        if ($pCell === null) {
            return self::REF();
        }

        return substr($pCell->getWorksheet()->getCell($value)->getValue(), 0, 1) === '=';
    }
public static function isFormula($value = '', Cell $pCell = null)
    {
        if ($pCell === null) {
            return self::REF();
        }

        return substr($pCell->getWorksheet()->getCell($value)->getValue(), 0, 1) === '=';
    }
ISFORMULA.

@param mixed $value The cell to check
@param Cell $pCell The current cell (containing this formula)

@return bool|string