$objPHPExcel = new PHPExcel();
# Query result goes to array $result
# ...
$i = 1;
for ($r = 0; $r < count($result); $r++) {
      # Set $value1, $value2, $value3 from $result
      $objPHPExcel->setActiveSheetIndex(0)
            ->setCellValue("A$i", $value1)
            ->setCellValue("B$i", $value2)
            ->setCellValue("C$i", $value3);
      $i++;
}
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save("/path/to/exported_file.xlsx");

Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 71 bytes)