I've never really thought about it before but PHP has some very similar metaprogramming features.

For starters, the incredibly dangerous runkit extension (like importing Objective-C's runtime).

There's also the Reflection API, which provides introspection and is quite widely used.

PHP also has magic methods, such as __call(), which give the ability to handle calling a method that doesn't exist.

As others have mentioned about Objective-C, these are all nice tools to know about and understand, however using them is often a code smell and can lead to unmaintainable code.

PHP doesn't have anything like categories, which is a shame because they look really useful in cases (as long as you don't mind violating the SRP a little). PHP has traits, which are more similar to mixins in ruby than categories in Objective-C.

Oh, PHP also has eval().