Interfaces must be OO contracts without code. PHP already has the traits to implement shared methods. Mixing both wouldn't be of any help.
The function scope must be respected in all cases. Function without scopes and default "use all variables" would be confusing. If you have nested closures, where the scope ends. Closures would be a nightmare like JavaScript functions.
The new syntax is noisy. It adds more cognitive load to the currently available function invocation methods.
The existing libraries supporting pipelining do it fine without extending the language syntax.