Well, according to Soledad Pendaes, PHP will never have a (real) Rails-like framework because of some of the limitations with objects inherent to the language.
I know the title is harsh but it's so true. At least it is according to nowadays php's implementation of classes and objects, which do not permit to "reopen" and add or redefine new methods to an existing class definition, which is the basis upon Rails (and I presume lots more of applications and frameworks) is built.
The post goes on to talk about they "why" behind the statement - basically that this kind of functionality makes it easier to "Extend. Morph. Adapt. Improve. Refine." your applications. She talks some about other frameworks that have tried to offer the same kinds of functionality Rails does (CakePHP and Symfony) but notes that they still have their limitations (in specific, mentioning CakePHP's AppModel functionality).