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/