docs(php): update migration guide for PHP (#935)

Author: damcou

website/docs/clients/guides/send-data-to-algolia.mdx

@@ -103,7 +103,7 @@ $records = [
 $requests = [];
 
 foreach ($records as $record) {
-    $requests += [
+    $requests[] = [
       'action' => 'addObject',
       'body' => $record
     ];

website/docs/clients/migration-guides/php.md

@@ -6,9 +6,10 @@ title: PHP
 |-----------------------------------------------|:---------------------------------------------------|:-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `"algolia/algoliasearch-client-php": "^3.2"`  | `"algolia/algoliasearch-client-php": "^4.0@alpha"` | **During the beta phase**, the clients are available under the package 4.x.x-alpha , you can find a full list [here](https://packagist.org/packages/algolia/algoliasearch-client-php).                                                               |
 | `Algolia\AlgoliaSearch`                       | `Algolia\AlgoliaSearch\Api`                        | Exported clients have now the namespace suffixed by `Api`.                                                                                                                                                                                           |
+| `Algolia\AlgoliaSearch\Support\UserAgent`     | `Algolia\AlgoliaSearch\Support\AlgoliaAgent`       | `UserAgent` class has been renamed to `AlgoliaAgent` for consistency across client languages (`addCustomUserAgent` method also became `addAlgoliaAgent`).                                                                                            |
+| `Algolia\AlgoliaSearch\SearchIndex`           | **removed**                                        | Since the method `initIndex` doesn't exist anymore, we decided to merge the `SearchIndex` class inside the `SearchClient` one, now all the methods related to search endpoints are located there.                                                    |
 | `Algolia\AlgoliaSearch\Cache\FileCacheDriver` | **removed**                                        | This implementation of the `CacheInterface` is not available anymore in the Client. If you feel the need for it, [please open an issue](https://github.com/algolia/api-clients-automation/issues/new?assignees=&labels=&template=Feature_request.md) |
 
-
 ### Usage
 
 To get started, first uninstall the previously added clients.
@@ -55,3 +56,23 @@ $searchResults2 = $client->search([
     ],
 ]);
 ```
+
+### Generate a secured Api Key 
+
+The `SearchClient::generateSecuredApiKey()` from the previous client was removed, but you can still create them:
+
+```php
+use Algolia\AlgoliaSearch\Support\Helpers;
+
+// Key will be valid for 25 hours.
+$validUntil = time() + (3600 * 25);
+
+$urlEncodedRestrictions = Helpers::buildQuery([
+    'restrictIndices' => '<YOUR_INDEX_NAME>',
+    'validUntil' => $validUntil,
+]);
+
+$content = hash_hmac('sha256', $urlEncodedRestrictions, '<YOUR_SEARCH_KEY>').$urlEncodedRestrictions;
+$securedSearchKey =  base64_encode($content);
+
+```