  Although run-time language composition is common, it normally takes the form
of a crude Foreign Function Interface (FFI). While useful, such compositions
tend to be coarse-grained and slow. In this paper we introduce a novel
fine-grained syntactic composition of PHP and Python which allows users to
embed each language inside the other, including referencing variables across
languages. This composition raises novel design and implementation challenges.
We show that good solutions can be found to the design challenges; and that the
resulting implementation imposes an acceptable performance overhead of, at
most, 2.6x.
