Tested only on Windows XP.
Open Program Files, and navigate to the Notepad++ directory (or whatever you called it upon installation).
Once in the Notepad++ directory, navigate to the plugins folder. You will see a list of .api files.
Go to the download folder of this MOD, and you will see a php.api file. Replace the php.api in the plugins directory with the one in this package.
Go to Notepad++ and open a PHP file. Press Ctrl+Spacebar and a list of functions will appear, all of those at the top are phpBB classes / functions (listed under phpBB Functions). All General PHP Functions are listed under General PHP Functions. After selecting the desired function from the list, either double click it, or select it and press enter, for it to appear in your PHP document.
Download this MOD from the phpBB.com MOD Database.