minor #10308 Encourage use of constants (trsteel88)

This PR was submitted for the 4.1 branch but it was squashed and merged into the 2.8 branch instead (closes #10308).

Discussion
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Encourage use of constants

Function should be static and we should also encourage the use of constants

Commits
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10422dc Encourage use of constants

Author: xabbuh

doctrine/event_listeners_subscribers.rst

@@ -179,8 +179,8 @@ interface and have an event method for each event it subscribes to::
         public function getSubscribedEvents()
         {
             return array(
-                'postPersist',
-                'postUpdate',
+                \Doctrine\ORM\Events::postPersist => 'postPersist',
+                \Doctrine\ORM\Events::postUpdate => 'postUpdate',
             );
         }