minor #3555 Wrap variables in {} for safer interpolation (ifdattic)

This PR was merged into the 2.3 branch.

Discussion
----------

Wrap variables in {} for safer interpolation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets |

Commits
-------

4046476 Replace double quotes with single quotes
0f7a189 Change interpolation to concatenation
d3839ce Wrap variables in {} for safer interpolation

components/http_foundation/sessions.rst

@@ -28,7 +28,7 @@ Quick example::
 
     // retrieve messages
     foreach ($session->getFlashBag()->get('notice', array()) as $message) {
-        echo "<div class='flash-notice'>$message</div>";
+        echo '<div class="flash-notice">'.$message.'</div>';
     }
 
 .. note::
@@ -308,18 +308,18 @@ Simple, display one type of message::
 
     // display warnings
     foreach ($session->getFlashBag()->get('warning', array()) as $message) {
-        echo "<div class='flash-warning'>$message</div>";
+        echo '<div class="flash-warning">'.$message.'</div>';
     }
 
     // display errors
     foreach ($session->getFlashBag()->get('error', array()) as $message) {
-        echo "<div class='flash-error'>$message</div>";
+        echo '<div class="flash-error">'.$message.'</div>';
     }
 
 Compact method to process display all flashes at once::
 
     foreach ($session->getFlashBag()->all() as $type => $messages) {
         foreach ($messages as $message) {
-            echo "<div class='flash-$type'>$message</div>\n";
+            echo '<div class="flash-'.$type.'">'.$message.'</div>';
         }
     }