How to use PHPSpreadsheet in CodeIgniter 3 to read data from Excel (.xlsx and .xls) file?
The easiest way to include PhpSpreadsheet is to use Composer - the documentation at https://phpspreadsheet.readthedocs.io/en/latest/ explains how to do this.
Otherwise you will need a PSR4 autoloader to load the class specified by your use statement or to include the files yourself. If you include files yourself, to read an Excel file you will need to include PhpSpreadsheet/src/PhpSpreadsheet/IOFactory.php