Add doc on how to add PHP extensions + details on the server root (#1169

)

docs/pages/docs/providers/php.md

@@ -10,6 +10,21 @@ Note that Laravel apps need an `APP_KEY` environment variable in order to work.
 
 If an `nginx.conf` or `nginx.template.conf` (see [this file](https://github.com/railwayapp/nixpacks/blob/main/src/providers/php/nginx.template.conf) for an example of template syntax) file is found in the project root directory, that configuration will be used.
 
+By default, the server root is set at your project root, which is mounted in `/app`. If your application is supposed to have its server root in a subfolder, you must set it using `NIXPACKS_PHP_ROOT_DIR` variable (see below). For example, if you're using Symfony or Laravel, the server root directory is `public/`; you then need to set `NIXPACKS_PHP_ROOT_DIR=/app/public`.
+
+To install PHP extensions, you can add them to the `require` section of your `composer.json`, following the [Composer platform dependencies specification](https://getcomposer.org/doc/articles/composer-platform-dependencies.md#different-types-of-platform-packages). The example below will add `ctype`, `iconv`, and `redis` extensions for PHP 8.2:
+
+```json
+{
+    "require": {
+        "php": ">=8.2",
+        "ext-ctype": "*",
+        "ext-iconv": "*",
+        "ext-redis": "*"
+    }
+}
+```
+
 If a `NIXPACKS_PHP_ROOT_DIR` variable is passed, that will be used as the server root.
 If a `NIXPACKS_PHP_FALLBACK_PATH` variable is passed, that will be used as a fallback for the server - for instance, if your app uses `index.php` as a router, you would set this variable to `/index.php`.