PHPOffice / PhpSpreadsheet

                $expectingOperator = false;

            } elseif ($opCharacter == ')' && $expectingOperator) {            //    Are we expecting to close a parenthesis?

                $expectingOperand = false;

                while (($o2 = $stack->pop()) && $o2['value'] != '(') {        //    Pop off the stack back to the last (

                    if ($o2 === null) {

                        return $this->raiseFormulaError('Formula Error: Unexpected closing brace ")"');

                    } else {

                        $output[] = $o2;

                    }

                }

                $d = $stack->last(2);

                if (preg_match('/^' . self::CALCULATION_REGEXP_FUNCTION . '$/i', $d['value'], $matches)) {    //    Did this parenthesis just close a function?

                    $functionName = $matches[1]; //    Get the function name

                }

                ++$index;

            } elseif ($opCharacter == ',') {            //    Is this the separator for function arguments?

                while (($o2 = $stack->pop()) && $o2['value'] != '(') {        //    Pop off the stack back to the last (

                    if ($o2 === null) {

                        return $this->raiseFormulaError('Formula Error: Unexpected ,');

                    } else {

                        $output[] = $o2; // pop the argument expression stuff and push onto the output

                    }

                }

                //    If we've a comma when we're expecting an operand, then what we actually have is a null operand;

                //        so push a null onto the stack

                if (($expectingOperand) || (!$expectingOperator)) {

            }

        }

        while (($op = $stack->pop()) !== null) {    // pop everything off the stack and push onto output

            if ((is_array($op) && $op['value'] == '(') || ($op === '(')) {

                return $this->raiseFormulaError("Formula Error: Expecting ')'"); // if there are any opening braces on the stack, then braces were unbalanced

            }

            $output[] = $op;

        }

        return $output;

    }