Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Reader/Xls.php
private function readFormula()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

        // move stream pointer to next record
        $this->pos += 4 + $length;

        // offset: 0; size: 2; row index
        $row = self::getUInt2d($recordData, 0);

        // offset: 2; size: 2; col index
        $column = self::getUInt2d($recordData, 2);
        $columnString = Coordinate::stringFromColumnIndex($column + 1);

        // offset: 20: size: variable; formula structure
        $formulaStructure = substr($recordData, 20);

        // offset: 14: size: 2; option flags, recalculate always, recalculate on open etc.
        $options = self::getUInt2d($recordData, 14);

        // bit: 0; mask: 0x0001; 1 = recalculate always
        // bit: 1; mask: 0x0002; 1 = calculate on open
        // bit: 2; mask: 0x0008; 1 = part of a shared formula
        $isPartOfSharedFormula = (bool) (0x0008 & $options);

        // WARNING:
        // We can apparently not rely on $isPartOfSharedFormula. Even when $isPartOfSharedFormula = true
        // the formula data may be ordinary formula data, therefore we need to check
        // explicitly for the tExp token (0x01)
        $isPartOfSharedFormula = $isPartOfSharedFormula && ord($formulaStructure[2]) == 0x01;

        if ($isPartOfSharedFormula) {
            // part of shared formula which means there will be a formula with a tExp token and nothing else
            // get the base cell, grab tExp token
            $baseRow = self::getUInt2d($formulaStructure, 3);
            $baseCol = self::getUInt2d($formulaStructure, 5);
            $this->baseCell = Coordinate::stringFromColumnIndex($baseCol + 1) . ($baseRow + 1);
        }

        // Read cell?
        if (($this->getReadFilter() !== null) && $this->getReadFilter()->readCell($columnString, $row + 1, $this->phpSheet->getTitle())) {
            if ($isPartOfSharedFormula) {
                // formula is added to this cell after the sheet has been read
                $this->sharedFormulaParts[$columnString . ($row + 1)] = $this->baseCell;
            }

            // offset: 16: size: 4; not used

            // offset: 4; size: 2; XF index
            $xfIndex = self::getUInt2d($recordData, 4);

            // offset: 6; size: 8; result of the formula
            if ((ord($recordData[6]) == 0) && (ord($recordData[12]) == 255) && (ord($recordData[13]) == 255)) {
                // String formula. Result follows in appended STRING record
                $dataType = DataType::TYPE_STRING;

                // read possible SHAREDFMLA record
                $code = self::getUInt2d($this->data, $this->pos);
                if ($code == self::XLS_TYPE_SHAREDFMLA) {
                    $this->readSharedFmla();
                }

                // read STRING record
                $value = $this->readString();
            } elseif ((ord($recordData[6]) == 1)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Boolean formula. Result is in +2; 0=false, 1=true
                $dataType = DataType::TYPE_BOOL;
                $value = (bool) ord($recordData[8]);
            } elseif ((ord($recordData[6]) == 2)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Error formula. Error code is in +2
                $dataType = DataType::TYPE_ERROR;
                $value = Xls\ErrorCode::lookup(ord($recordData[8]));
            } elseif ((ord($recordData[6]) == 3)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Formula result is a null string
                $dataType = DataType::TYPE_NULL;
                $value = '';
            } else {
                // forumla result is a number, first 14 bytes like _NUMBER record
                $dataType = DataType::TYPE_NUMERIC;
                $value = self::extractNumber(substr($recordData, 6, 8));
            }

            $cell = $this->phpSheet->getCell($columnString . ($row + 1));
            if (!$this->readDataOnly) {
                // add cell style
                $cell->setXfIndex($this->mapCellXfIndex[$xfIndex]);
            }

            // store the formula
            if (!$isPartOfSharedFormula) {
                // not part of shared formula
                // add cell value. If we can read formula, populate with formula, otherwise just used cached value
                try {
                    if ($this->version != self::XLS_BIFF8) {
                        throw new Exception('Not BIFF8. Can only read BIFF8 formulas');
                    }
                    $formula = $this->getFormulaFromStructure($formulaStructure); // get formula in human language
                    $cell->setValueExplicit('=' . $formula, DataType::TYPE_FORMULA);
                } catch (PhpSpreadsheetException $e) {
                    $cell->setValueExplicit($value, $dataType);
                }
            } else {
                if ($this->version == self::XLS_BIFF8) {
                    // do nothing at this point, formula id added later in the code
                } else {
                    $cell->setValueExplicit($value, $dataType);
                }
            }

            // store the cached calculated value
            $cell->setCalculatedValue($value);
        }
    }
private function readFormula()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

        // move stream pointer to next record
        $this->pos += 4 + $length;

        // offset: 0; size: 2; row index
        $row = self::getUInt2d($recordData, 0);

        // offset: 2; size: 2; col index
        $column = self::getUInt2d($recordData, 2);
        $columnString = Coordinate::stringFromColumnIndex($column + 1);

        // offset: 20: size: variable; formula structure
        $formulaStructure = substr($recordData, 20);

        // offset: 14: size: 2; option flags, recalculate always, recalculate on open etc.
        $options = self::getUInt2d($recordData, 14);

        // bit: 0; mask: 0x0001; 1 = recalculate always
        // bit: 1; mask: 0x0002; 1 = calculate on open
        // bit: 2; mask: 0x0008; 1 = part of a shared formula
        $isPartOfSharedFormula = (bool) (0x0008 & $options);

        // WARNING:
        // We can apparently not rely on $isPartOfSharedFormula. Even when $isPartOfSharedFormula = true
        // the formula data may be ordinary formula data, therefore we need to check
        // explicitly for the tExp token (0x01)
        $isPartOfSharedFormula = $isPartOfSharedFormula && ord($formulaStructure[2]) == 0x01;

        if ($isPartOfSharedFormula) {
            // part of shared formula which means there will be a formula with a tExp token and nothing else
            // get the base cell, grab tExp token
            $baseRow = self::getUInt2d($formulaStructure, 3);
            $baseCol = self::getUInt2d($formulaStructure, 5);
            $this->baseCell = Coordinate::stringFromColumnIndex($baseCol + 1) . ($baseRow + 1);
        }

        // Read cell?
        if (($this->getReadFilter() !== null) && $this->getReadFilter()->readCell($columnString, $row + 1, $this->phpSheet->getTitle())) {
            if ($isPartOfSharedFormula) {
                // formula is added to this cell after the sheet has been read
                $this->sharedFormulaParts[$columnString . ($row + 1)] = $this->baseCell;
            }

            // offset: 16: size: 4; not used

            // offset: 4; size: 2; XF index
            $xfIndex = self::getUInt2d($recordData, 4);

            // offset: 6; size: 8; result of the formula
            if ((ord($recordData[6]) == 0) && (ord($recordData[12]) == 255) && (ord($recordData[13]) == 255)) {
                // String formula. Result follows in appended STRING record
                $dataType = DataType::TYPE_STRING;

                // read possible SHAREDFMLA record
                $code = self::getUInt2d($this->data, $this->pos);
                if ($code == self::XLS_TYPE_SHAREDFMLA) {
                    $this->readSharedFmla();
                }

                // read STRING record
                $value = $this->readString();
            } elseif ((ord($recordData[6]) == 1)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Boolean formula. Result is in +2; 0=false, 1=true
                $dataType = DataType::TYPE_BOOL;
                $value = (bool) ord($recordData[8]);
            } elseif ((ord($recordData[6]) == 2)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Error formula. Error code is in +2
                $dataType = DataType::TYPE_ERROR;
                $value = Xls\ErrorCode::lookup(ord($recordData[8]));
            } elseif ((ord($recordData[6]) == 3)
                && (ord($recordData[12]) == 255)
                && (ord($recordData[13]) == 255)) {
                // Formula result is a null string
                $dataType = DataType::TYPE_NULL;
                $value = '';
            } else {
                // forumla result is a number, first 14 bytes like _NUMBER record
                $dataType = DataType::TYPE_NUMERIC;
                $value = self::extractNumber(substr($recordData, 6, 8));
            }

            $cell = $this->phpSheet->getCell($columnString . ($row + 1));
            if (!$this->readDataOnly) {
                // add cell style
                $cell->setXfIndex($this->mapCellXfIndex[$xfIndex]);
            }

            // store the formula
            if (!$isPartOfSharedFormula) {
                // not part of shared formula
                // add cell value. If we can read formula, populate with formula, otherwise just used cached value
                try {
                    if ($this->version != self::XLS_BIFF8) {
                        throw new Exception('Not BIFF8. Can only read BIFF8 formulas');
                    }
                    $formula = $this->getFormulaFromStructure($formulaStructure); // get formula in human language
                    $cell->setValueExplicit('=' . $formula, DataType::TYPE_FORMULA);
                } catch (PhpSpreadsheetException $e) {
                    $cell->setValueExplicit($value, $dataType);
                }
            } else {
                if ($this->version == self::XLS_BIFF8) {
                    // do nothing at this point, formula id added later in the code
                } else {
                    $cell->setValueExplicit($value, $dataType);
                }
            }

            // store the cached calculated value
            $cell->setCalculatedValue($value);
        }
    }
Read FORMULA record + perhaps a following STRING record if formula result is a string
This record contains the token array and the result of a
formula cell.

--    "OpenOffice.org's Documentation of the Microsoft
Excel File Format"