Expressions executed in the `Context.Eval` method can now return a
value, which will be passed to the Go caller, and can be used as a
regular Go value.
Call order is now enforced when using the root `php` package by moving
the `NewContext` function in the `engine` package, as a method of the
`Engine` method receiver. Further clean-ups have been made to the code
as a result of this change.