isis has been used in a number of popular commercial and private websites and has proven its self to be extreamly scaleable.
isis is a collection of PHP classes that aid in rapid development of websites and online applications.
isis is not a framework, however a simple framework can be built with only a few lines of code, using the isisSite class, or the supplied command line tool.
isis was started in 2003 to bring together a collection of classes developed by Chris Hobden over the previous 5 years. Over time this has expanded into library avalible today.
In 2005 isis was incorporated into the Brutus framework, also developed by Chris Hobden. The framework, whitest advanced, was discontinued in 2007 in favour of isisSite.
In early 2008 version 2, codenamed Philae, of isis was branched from the original code in an effort to shed some of the shackles laid down by such a long development line.
The isis library tries to remain as un-intrusive as possible, letting you plug it into an existing site or build a site from scratch with only a few lines of code. The classes all live in separate files but only one minimal file needs to be included in your code. Nothing else is read or loaded into memory unless used, this means even though there are tens of thousands of lines in the libraries if you only use one or two classes you don't have to worry about the entire library getting parsed every page load.
This is all handled automatically for you by the libraries, you never need to remember to include classes in your code. All the classes inherit from each other and try to use very similar syntax, an inheritce table is avalible in the documentaion.