[edit] Follow the standard route...

Introduction-Understand the background of phpBB before we start

then

Getting started-To understand the code of phpBB or to create or modify the same, you need some tools to aid you

then

Basic anaylsis-A grand overview on the structure of phpBB and some basics

then

Understanding the basic classes-phpBB has a number of classes to handle common tasks, it's important that you understand them because they appear frequently

then

Detailed anaylsis file by file with
Function reference guide-Go into the core of anaylsising, with function guide to help when needed

then

Common tasks-Learn how common tasks are done in phpBB, with code example

and then...

Congratulation! But! Is it the end?
No! If you get this far, we encourage you to try writing code together with our community! Maybe our codes will never be used, but this is fun! Coding phpBB tour

[edit] Or browse directly

Frontend Files:

download.php, faq.php, index.php, mcp.php, memberlist.php, posting.php, report.php, search.php, ucp.php, viewforum.php, viewonline.php, viewtopic.php

Backend Files:

common.php, config.php, cron.php, style.php

includes/

bbcode.php, functions.php, functions_admin.php, functions_compress.php, functions_display.php, functions_jabber.php, functions_messenger.php, functions_module.php, functions_posting.php, functions_privmsgs.php, functions_profile_fields.php, functions_upload.php, functions_user.php, message_parser.php, session.php, template.php

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