When you look at Excel solutions for PHP, the first thing you find is PHPExcel, digging a little bit further lets you find libXl. The prior is a pure PHP solution, the latter a native library for a couple of operating systems. There is a PHP extension that exposes this library to PHP, so you can do the same things with it that you can do with PHPExcel.
TL;DR: PHPExcel is nice, libXl with php_excel can do the same WAY faster. Use the Diagrams below to convince your customer/boss/CFO to invest in performance.