It's fairly common in languages that determine (return) types themselves, e.g. Haskell, Scala and F# all use this syntax. Most languages that use a C-style syntax put (return) types in front.

It makes very little sense for PHP. A function definition now even has both styles:

`function foo(argtype arg) : rettype` instead of `function rettype foo(argtype arg)` or `function foo(arg : argtype) : rettype`.

Hack, ActionScript and TypeScript put the type after. Hack is most likely where this syntax came from.