What do you do with higher-order functions (which PHP has I believe)? If I write a typed map : (a -> b) -> [a] -> [b] and then it's called with an untyped func

What do you do with higher-order functions (which PHP has I believe)? If I write a typed map : (a -> b) -> [a] -> [b] and then it's called with an untyped function as input, but it returns a something that's not a b. Do you place some kind of barrier around it so that that's stopped immediately?