phpBB 3 features a cache system that creates static copies of phpBB files - primarily the ones related to the phpBB styles that you have installed. This is a good idea because it makes your site load faster and it reduces the amount of bandwidth that your phpBB uses, which saves you money and aggravation.
But, when it comes to installing phpBB hacks or making adjustments to your phpBB style(s), one of the most common reasons that something doesn't work or a change isn't reflected is because your phpBB is displaying an old, cached version of a file that you changed. For the change to take effect, you may need to clear your cache.
How do you do this? The simplest way is to go to your phpBB 3 admin panel and on the index page of that area, you will see your "Board statistics." Below that is an area titled "Resynchronise or reset statistics." This area features a series of "Run now" buttons. Look for the one next to "Purge the cache." Press this button, confirm your decision and your cache has been fully purged. Generally speaking, this will be perfect for most people looking to clear their cache.
Clearing the Template Files, Theme Files or Imagesets For a Specific Style
However, you may want to clear the cache for the specific template files, theme files or imagesets associated with a given style. To do this, once in the admin area, click on the "Styles" tab at the top. From there, you'll see "Templates," "Themes" and "Imagesets" in the left menu. Click the one you would like to clear.
You will now see a list of the styles you have installed. Under "Actions," click "Refresh" next to the one whose cache you wish to clear, confirm your decision and it has been cleared.
Making Major Changes? Tell phpBB 3 to Check if a File Was Updated
Finally, if you are making a number of changes, you can tell phpBB to check to see if a new version of a file has been uploaded, before it uses the cached one.
On your phpBB 3 admin panel index page, in the the left menu, under "Server Configuration," click on "Load settings." On the next page, at the bottom of "General options," you'll see "Recompile stale style components." Select "Yes" and press the "Submit" button at the bottom.
This is generally recommended only when you are making a bunch of changes because it can cause increased load on your server. After you have made your changes, you should switch this option back to "No."
Hopefully this tutorial has shown you the ways of the phpBB 3 cache. Thank you to Robert (lumpy burgertushie) and ~HG~ for the inspiration.