Adds example for generating Cloud CDN Signed URLs

Example implementation of generating URL signature for Cloud CDN.
Based on: https://cloud.google.com/cdn/docs/using-signed-urls#creating_signed_urls

Author: brona

cdn/signUrl.php

@@ -0,0 +1,51 @@
+<?php
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+# [START signed_url]
+/**
+ * Creates signed URL for Google Cloud CDN
+ * Details about order of operations: https://cloud.google.com/cdn/docs/using-signed-urls#creating_signed_urls
+ *
+ * @param string $url             URL of the endpoint served by Cloud CDN
+ * @param string $keyName         Name of the signing key added to the Google Cloud Storage bucket or service
+ * @param string $key             Signing key as base64 encoded string
+ * @param int    $expiration_time Expiration time as a UNIX timestamp (GMT, e.g. time())
+ */
+function signUrl($url, $keyName, $key, $expiration_time)
+{
+    // Decode the key
+    $decoded_key = base64_decode($key, true);
+
+    // Determine which separator makes sense given a URL
+    $separator = (strpos($url, '?') === false) ? '?' : '&';
+
+    // Concatenate url with expected query parameters Expires and KeyName
+    $url = "{$url}{$separator}Expires={$expiration_time}&KeyName={$keyName}";
+
+    // Sign the url using the key and encode the signature using base64
+    $signature = hash_hmac('sha1', $url, $decoded_key, true);
+    $encoded_signature = base64_encode($signature);
+
+    // Concatenate the URL and encoded signature
+    return "{$url}&Signature={$encoded_signature}";
+}
+// [END signed_url]
+
+// Example function call (In production store the key safely with other secrets)
+$base64_key = '4RfgBoqmotRolLCtU-82Ew=='; // head -c 16 /dev/urandom | base64 | tr +/ -_
+$signed_url = signUrl('https://example.com/foo', 'MY-KEY', $base64_key, time() + 1800);
+echo $signed_url."\n";