third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/php/README.md

+This directory contains the Protocol Buffers runtime implementation via both a
+pure PHP package and a native c extension. The pure PHP package is intended to
+provide usability to wider range of PHP platforms, while the c extension is
+intended to provide higher performance. Both implementations provide the same
+runtime APIs and share the same generated code. Users don’t need to re-generate
+code for the same proto definition when they want to switch the implementation
+later.
+
+Both implementations make use of generated PHP code that defines message and
+enum types in PHP. We strongly recommend using protoc's PHP generation support
+with .proto files. The build process in this directory only installs the
+extension/package; you need to install protoc as well to have PHP code
+generation functionality.
+
+## Requirements
+
+To use PHP runtime library requires:
+
+- C extension: PHP 5.5.x or 5.6.x.
+- PHP package: PHP 5.5, 5.6 or 7.
+
+## Installation
+
+### C Extension
+
+#### Prerequirements
+
+To install the c extension, the following tools are needed:
+* autoconf
+* automake
+* libtool
+* make
+* gcc
+* pear
+* pecl
+
+On Ubuntu, you can install them with:
+```
+sudo apt-get install php-pear php5-dev autoconf automake libtool make gcc
+```
+On other platforms, please use the corresponding package managing tool to
+install them before proceeding.
+
+#### Installation from Source (Building extension)
+
+To build the c extension, run the following command:
+```
+cd ext/google/protobuf
+pear package
+sudo pecl install protobuf-{VERSION}.tgz
+```
+
+#### Installation from PECL
+
+When we release a version of Protocol Buffers, we will upload the extension to
+[PECL](https://pecl.php.net/). To use this pre-packaged extension, simply
+install it as you would any other extension:
+
+```
+sudo pecl install protobuf-{VERSION}
+```
+
+### PHP Package
+
+#### Installation from composer
+
+Simply add "google/protobuf" to the 'require' section of composer.json in your
+project.
+
+### Protoc
+
+Once the extension or package is installed, if you wish to generate PHP code
+from a `.proto` file, you will also want to install the Protocol Buffers
+compiler (protoc), as described in this repository's main `README` file.  The
+version of `protoc` included in the latest release supports the `--php_out`
+option to generate PHP code:
+```
+protoc --php_out=out_dir test.proto
+```
+
+## Usage
+
+For general guide:
+  https://developers.google.com/protocol-buffers/phptutorial/
+For generated code:
+  https://developers.google.com/protocol-buffers/docs/reference/php-generated
+
+Known Issues
+------------
+
+* Missing native support for well known types.
+* Missing support for proto2.
+* No API provided for clear/copy messages.
+* No API provided for encoding/decoding with stream.
+* Map fields may not be garbage-collected if there is cycle reference.
+* No debug information for messages in c extension.
+* HHVM not tested.
+* C extension not tested on windows, mac, php 7.0.
+* Message name cannot be Empty.