Adjective
dynamically-typed (not comparable)
(computing) belonging to a type system that checks the variable types at runtime.
For example, C# distinguishes between "statically-typed" and "dynamically-typed" variables; uses of the former are checked statically, while uses of the latter are checked dynamically.
You may not only change the content of a variable in a dynamically-typed language like PHP on the fly, but its type as well.