Translations:Extension:Scribunto/Lua reference manual/2291/en

Additional caveats
Such data structures may be used freely within Lua, including as the return values of modules loaded with.
 * Referential data structures: Circular data structures and data structures where the same node may be reached by more than one path cannot be correctly sent to PHP. Attempting to do so will cause undefined behavior. This includes (but is not limited to) returning such data structures from the module called by  and passing such data structures as parameters to Scribunto library functions that are implemented as callbacks into PHP.