PHPOffice/PHPExcel ? reverse-engineered prompt

I have an old PHP app that uses PHPExcel and I want to modernize it without breaking the spreadsheet features.

Please help me replace the old Excel handling with the current supported library and make sure the app can still open, edit, and save common spreadsheet files like Excel and CSV. I need imports to read rows into PHP cleanly, exports to generate files with basic formatting, and support for things like multiple sheets, formulas, dates, and merged cells if possible.

Keep the code simple and easy to drop into an existing PHP project, and update any outdated setup or examples so I can follow along. If anything from the old library works differently now, adjust the code and explain the changes in plain language. Look up current docs online if you need to.