On Medium.com Dylan Bridgman has posted a tutorial with a few tips helping you improve the perceived load time of your application, the time between when the page starts loading and the user can first do something with its contents.
In this article I will concentrate on perceived load time. That is the time from when a page starts loading until the user is able to proceed. There have been other articles on this topic, some of which go into far more detail in some areas. My intention is to give a summary on the latest information and what you should be doing in PHP. I will link to additional information where applicable.
He breaks the advice up into three main categories: output buffering, multiple connections and using correctly structured pages. Under each section there's a few sub-points with more information, details on implementation and links to other additional information. He also includes an example that combines all of the advice into one PHP/HTML script outputting some basic content.