Just like with PHPExcel, PHPPowerPoint can be used to generate PPTX files from a PHP application. This can be done by creating an in-memory presentation that consists of slides and different shapes, which can then be written to disk using a writer (of which there's currently only one for PowerPoint 2007).
Some example code is included that creates a single-slide slideshow with the PHPPowerpoint logo and the text "Thank you for using PHPPowerpoint". There's plenty of configuration options and you can place things in the slides down to the pixel. You can try out this initial release here.