Check in php implementation. (#2052)

This pull request includes two implementation: C extension and PHP
package. Both implementations support encode/decode of singular,
repeated and map fields.
diff --git a/composer.json b/composer.json
new file mode 100644
index 0000000..8b73a2b
--- /dev/null
+++ b/composer.json
@@ -0,0 +1,24 @@
+{
+  "name": "google/protobuf",
+  "type": "library",
+  "description": "proto library for PHP",
+  "keywords": ["proto"],
+  "homepage": "https://developers.google.com/protocol-buffers/",
+  "license": "BSD-3-Clause",
+  "require": {
+    "php": ">=5.5.0"
+  },
+  "require-dev": {
+    "phpunit/phpunit": ">=4.8.0"
+  },
+  "autoload": {
+    "psr-4": {
+      "Google\\": "php/src/Google"
+    },
+    "files": [
+      "php/src/Google/Protobuf/descriptor.php",
+      "php/src/Google/Protobuf/descriptor_internal.pb.php",
+      "php/src/Google/Protobuf/Internal/Type.php"
+    ]
+  }
+}