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Objective-PHP is a port of the Objective-C (or Objective-J runtime to PHP. This adds the language features of Objective-C nestled nicely inside the syntax of Objective-C. Moka is a port of the Apple Cocoa Frameworks (or Cappuccino).

Useful Links and Info

Objective-C Object Model

http://www.sealiesoftware.com/blog/archive/2009/04/14/objcexplainClassesandmetaclasses.html

C source of Objective-C runtime

http://www.opensource.apple.com/source/objc4/objc4-274/

http://www.opensource.apple.com/source/gcc/gcc-1640/libobjc/