minor #9493 Document host parameter and external parameters in "How to Use PdoSessionHandler..." (pueppiblue, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #9493).

Discussion
----------

Document host parameter and external parameters in "How to Use PdoSessionHandler..."

I recently had the use case to store sessions via pdo on a docker grid.
I noticed the `doctrine/pdo_session_storage.rst` documentation is missing the option
to configure a host on the PdoSessionHandler and that
the service configuration does not use external environment parameters.

  - add host parameter to first argument
    useful when database is on another host e.g. docker grid
  - wrap parameter values in %env()%
  - add note how to use external parameters

Commits
-------

d49d8a7 Remove some unneeded changes
a7c3344 Don't use env vars in all config examples
8ea2380 add more trailing comma on last array element #9493
ba5fa52 add trailing comma on last array element #9493
e291746 codestyle
cd24113 add documentation for external parameters and pdo host parameter

Author: javiereguiluz

doctrine/pdo_session_storage.rst

@@ -24,7 +24,7 @@ To use it, first register a new handler service:
             Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler:
                 public:    false
                 arguments:
-                    - 'mysql:dbname=mydatabase'
+                    - 'mysql:dbname=mydatabase, host=myhost'
                     - { db_username: myuser, db_password: mypassword }
 
     .. code-block:: xml
@@ -40,7 +40,7 @@ To use it, first register a new handler service:
 
             <services>
                 <service id="Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler" public="false">
-                    <argument>mysql:dbname=mydatabase</argument>
+                    <argument>mysql:dbname=mydatabase, host=myhost</argument>
                     <argument type="collection">
                         <argument key="db_username">myuser</argument>
                         <argument key="db_password">mypassword</argument>
@@ -56,11 +56,17 @@ To use it, first register a new handler service:
 
         $storageDefinition = $container->register(PdoSessionHandler::class)
             ->setArguments(array(
-                'mysql:dbname=mydatabase',
-                array('db_username' => 'myuser', 'db_password' => 'mypassword')
+                'mysql:dbname=mydatabase, host=myhost',
+                array('db_username' => 'myuser', 'db_password' => 'mypassword'),
             ))
         ;
 
+.. tip::
+
+    Configure the database credentials as
+    :doc:`parameters defined with environment variables </configuration/external_parameters>`
+    to make your application more secure.
+
 Next, tell Symfony to use your service as the session handler:
 
 .. configuration-block::
@@ -113,8 +119,8 @@ a second array argument to ``PdoSessionHandler``:
             Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler:
                 public:    false
                 arguments:
-                    - 'mysql:dbname=mydatabase'
-                    - { db_table: sessions, db_username: myuser, db_password: mypassword }
+                    - 'mysql:dbname=mydatabase, host=myhost'
+                    - { db_table: 'sessions', db_username: 'myuser', db_password: 'mypassword' }
 
     .. code-block:: xml
 
@@ -127,7 +133,7 @@ a second array argument to ``PdoSessionHandler``:
 
             <services>
                 <service id="Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler" public="false">
-                    <argument>mysql:dbname=mydatabase</argument>
+                    <argument>mysql:dbname=mydatabase, host=myhost</argument>
                     <argument type="collection">
                         <argument key="db_table">sessions</argument>
                         <argument key="db_username">myuser</argument>
@@ -146,7 +152,7 @@ a second array argument to ``PdoSessionHandler``:
 
         $container->register(PdoSessionHandler::class)
             ->setArguments(array(
-                'mysql:dbname=mydatabase',
+                'mysql:dbname=mydatabase, host=myhost',
                 array('db_table' => 'sessions', 'db_username' => 'myuser', 'db_password' => 'mypassword')
             ))
         ;