minor #15010 Minor change to use DateTimeImmutable instead of DateTime (Erdou)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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Minor change to use DateTimeImmutable instead of DateTime

Using DateTimeImmutable instead of DateTime when no mutation is needed improves performances.

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Commits
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8c6912b Minor change to use DateTimeImmutable instead of DateTime

Author: javiereguiluz

doctrine/events.rst

@@ -76,7 +76,7 @@ define a callback for the ``prePersist`` Doctrine event:
              */
             public function setCreatedAtValue(): void
             {
-                $this->createdAt = new \DateTime();
+                $this->createdAt = new \DateTimeImmutable();
             }
         }