Many people are familiar with the phpinclude_start and phpinclude_end templates of vBulletin 3. You could use these to include PHP in 2 or 3 places in vBulletin. It was not useful for extending control panels, manipulating users, threads, posts, etc. vBulletin 3.5 introduced Plugins which allow you to introduce PHP code into vBulletin in over 500 key areas or hooks. This allows you to extend vBulletin in dramatic ways with relatively simple code. More information on writing plugins and products is available at vBulletin.org.