Joel on Software has a good article that explains functors, the purpose of passing functions as paramters, and leads into an explanation of map/reduce and parallel computing. These ideas are explained in Javascript, but also apply to Lisp, Scheme, and Perl (see the Mark Jason Dominus book). All these languages are functional programming languages, where functions are objects (or objects are functions).

(Add examples of functional programming here.)

Unfortunately, PHP, the other language covered here, has only half-baked functional programming. See the Function Handling Functions section of the manual.