What is the best way to do this...

Let s say i have 5 pages on my site. I want some content to appear on each page so that when i want to change that content on all the pages i have only one place to modify. I used to write that content in a .txt file and having it read each time any of the 5 pages was loaded. So i had to modify the code only in the .txt file. It was working good but the code in the .txt file could not be PHP because it would not be processed when loaded by the other pages. There s probably a better way of doing it. Like a page can probably include another page that itself include another one. Any help would be great Thx