Memory Efficient reading + writing of .csv and .xlsx data - mysql

Question

I am looking to implement memory efficient reading and writing of .xlsx/.csv files.
I am currently using phpExcel and am having trouble with large files. (Known issue)
I found the following library for reading: https://github.com/nuovo/spreadsheet-reader
This seems that it will work.
For writing files currently I create an array and then loop the array to output the file. Is this why it uses a lot of memory? What is the best way to not use a lot of memory when writing csv/xlsx when getting data from mysql database using code igniter?
This is what I do now:
/* added for excel expert */
function excel_export() {
$data = $this->Customer->get_all($this->Customer->count_all())->result_object();
$this->load->helper('report');
$rows = array();
$header_row = $this->_excel_get_header_row();
$header_row[] = lang('customers_customer_id');
$rows[] = $header_row;
foreach ($data as $r) {
$row = array(
$r->first_name,
$r->last_name,
$r->email,
$r->phone_number,
$r->address_1,
$r->address_2,
$r->city,
$r->state,
$r->zip,
$r->country,
$r->comments,
$r->account_number,
$r->taxable ? 'y' : '',
$r->company_name,
$r->person_id
);
$rows[] = $row;
}
$content = array_to_spreadsheet($rows);
force_download('customers_export.'.($this->config->item('spreadsheet_format') == 'XLSX' ? 'xlsx' : 'csv'), $content);
exit;
}
function array_to_spreadsheet($arr)
{
$CI =& get_instance();
$objPHPExcel = new PHPExcel();
//Default all cells to text
$objPHPExcel->getDefaultStyle()->getNumberFormat()->setFormatCode(PHPExcel_Style_NumberFormat::FORMAT_TEXT);
for($k = 0;$k < count($arr);$k++)
{
for($j = 0;$j < count($arr[$k]); $j++)
{
$objPHPExcel->getActiveSheet()->setCellValueExplicitByColumnAndRow($j, $k+1, $arr[$k][$j]);
}
}
if ($CI->config->item('spreadsheet_format') == 'XLSX')
{
$objWriter = new PHPExcel_Writer_Excel2007($objPHPExcel);
}
else
{
$objWriter = new PHPExcel_Writer_CSV($objPHPExcel);
}
ob_start();
$objWriter->save('php://output');
$excelOutput = ob_get_clean();
return $excelOutput;
}