Add a script to mirror php changes to protobuf-php (#5778)

* Add a script to mirror php changes to protobuf-php

* Create tag

php/release.sh

+#!/bin/bash
+
+set -ex
+
+# Make sure we are in a protobuf source tree.
+[ -f "php/release.sh" ] || {
+  echo "This script must be ran under root of protobuf source tree."
+  exit 1
+}
+
+VERSION=$1
+
+git clone https://github.com/protocolbuffers/protobuf-php.git
+git clone https://github.com/protocolbuffers/protobuf.git
+
+# Clean old files
+pushd protobuf-php
+rm -rf src
+popd
+
+# Checkout the target version
+pushd protobuf/php
+git checkout $VERSION
+popd
+
+# Copy files
+pushd protobuf-php
+mv ../protobuf/php/src src
+mv ../protobuf/composer.json composer.json
+sed -i 's|php/src|src|g' composer.json
+git add .
+git commit -m "$VERSION"
+git tag "$VERSION"
+popd
+
+# Clean up
+rm -rf protobuf