How much code does it share with regular WordPress?
Basically WordPress MU is a wrapper around the core WP code that virtualizes multiple blogs. The code that differs is in some bootstrap files, and administration for multiple blogs and users. It’s tough to say exactly what percentage of the total code this comprises, but it’d be safe to say that 95-99% of MU is core WP.
MU was developed when Donncha wanted to have multiple blogs from one install, with multiple admins. Eventually, this was also the beginnings of WordPress.com, but that’s another story. We’ll focus on the original code for now.
The development MU closely follows WordPress nightly builds, with features being rolled into MU before they are even released to the general public. When you hear of a new WordPress release, most of the time it has already shown up in MU. At the same time, Donncha works on the MU-specific sections of code. You can follow his progress through the trac system.
With MU, each user is the admin of their very own blog, just like in WordPress. The site admin controls the main blog and has access to all the user’s blogs.
The major difference here are:
– The installation process is different.
– Users cannot upload plugins and themes as they have no ftp access to the server.
– The plugin editor and theme editor menu options are disabled.
– There is a Site Admin menu for the main site admin to control various aspects of how the site is run.
– A new folder, mu-plugins, is present for plugins that automatically run site-wide. [?]
– Table creation for each blog includes a blog id number in the prefix. This may cause regular plugins to not function properly if they write information to the database.
Since so much of the code is based on WordPress itself, there is an assumption that a WordPressMU site administrator is familiar with WordPress. If you are diving in for the first time, it would be in your best interest to play around with WordPress a bit first to figure out what behavior is default and normal.
WordPressMU is not well-suited for someone looking to run a handful of blogs, or someone not comfortable with editing code or working on the server level. On the other hand, if you are willing to learn and occasionally make mistakes, you may find running a WordPressMU powered site to be very enriching indeed.