The Lua standalone engine has a message-based protocol for communicating between
PHP and Lua.

Messages start with a 16 byte header. The first 8 bytes are the body length in
hexadecimal. The second 8 bytes are (length * 2 - 1) also in hexadecimal.

For messages passed from PHP to Lua, the body is encoded as a Lua expression.
The expression may reference a table in a variable called "chunks", which
contains an array of functions.

Messages passed from Lua to PHP have their body encoded in PHP serialize()
format, and then "\\", "\r", and "\n" are replaced with "\\\\", "\\r", and
"\\n" to avoid issues with text-mode file handles. They may include instances
of function objects which have an "id" member for passing back to Lua as an
index in the chunk table.

The expressions encoded into the message bodies are associative arrays. The "op"
member of the array gives the operation to be performed by the message.

Every request message demands exactly one response message in reply. When a
request message is sent, the responder does not need to send the corresponding
response message as its next message. It may instead send its own request
message. In this way, a stack of pending requests can be accumulated. This
mechanism allows re-entrant and recursive calls.

All numerically-indexed arrays should start from index 1 unless otherwise
specified. Note that the number of values in an array may not match what Lua's
'#' operator returns if the array contains nils.

== Request messages sent from PHP to Lua ==

=== loadString ===

Load some executable Lua code (a "chunk") and return the resulting function ID.

Message parameters:
* op: "loadString"
* text: The string to load
* chunkName: The name of the string, for use in error messages

On success, the response message is:

* op: "return"
* nvalues: 1
* values: An array with a single element with the ID in it

On failure, the response message is:

* op: "error"
* value: The error message

=== call ===

Call a Lua function.

Message parameters:
* op: "call"
* id: The chunk ID
* nargs: Number of arguments, including nils
* args: The argument array

On success, the response message is:

* op: "return"
* nvalues: Number of return values, including nils
* values: All return values as an array

On failure, the response message is:

* op: "error"
* value: The value given to error(), usually an error message
* trace: A table giving a backtrace of the stack as it was when error() was
  called, in a similar format to the one used by debug.getinfo(). Element 1 of
  the table is the function that called error(), element 2 is the function that
  called that, and so on.

=== registerLibrary ===

Register a set of functions in the sandbox environment.

Message parameters:
* op: "registerLibrary"
* name: The global variable name to register. May contain "." characters to
  specify a global variable subtable.
* functions: An associative array mapping function name to ID

On success, the response message is:

* op: "return"
* nvalues: 0
* values: An empty array

On failure the response message is:

* op: "error"
* value: The error message

=== getStatus ===

Get status information about the Lua process.

Message parameters:
* op: "getStatus"

On success, the response message is:

* op: "return"
* nvalues: 1
* values: An array with a single element, which is an associative array mapping
  status key to value. The status keys are:
** pid: The process identifier
** time: The amount of user and system time spent by the process, measured in clock ticks
** vsize: The virtual memory size in bytes
** rss: The resident set size in bytes

On failure, the response message is:

* op: "return"
* nvalues: 0
* values: An empty array

=== cleanupChunks ===

Tell Lua to release any chunks no longer referenced by PHP.

Message parameters:
* op: "cleanupChunks"
* ids: Table with keys being the chunk IDs still referenced by PHP, and non-falsey values

The response message is:

* op: "return"
* nvalues: 0
* values: An empty array

=== quit ===

Request graceful shutdown.

Message parameters:
* op: "quit"

No return message will be sent.

=== testquit ===

Request non-graceful shutdown, for testing.

Message parameters:
* op: "testquit"

No return message will be sent.

== Request messages sent from Lua to PHP ==

=== call ===

Call a PHP function.

Message parameters:
* op: "call"
* id: The function ID given by registerLibrary
* nargs: Number of arguments, including nils
* args: An array giving the function arguments

On success, the response message is:

* op: "return"
* nvalues: Number of return values, including nils
* values: All return values as an array

On failure the response message is:

* op: "error"
* value: The error message

