PHPExcel
Library offering classes to read from and write to spreadsheet formats
...Because it handles the details of Excel file structures—including relationships, themes, and various metadata—users don’t need to manage proprietary file internals themselves. The API allows for cell access using row/column indexes or column letters, batch updates, and streaming writes to reduce memory usage. As a mature library, it’s used in reporting, migrations, invoice generation, and any scenario where dynamic, server-side spreadsheet generation is needed in PHP applications.