feature #11956 Added small section about default values (j92)

This PR was merged into the 4.4 branch.

Discussion
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Added small section about default values

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Added a short description and a code sample of how to use default values when referenced variables are not set. The feature PR is symfony/symfony#31546.

Commits
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1848e00 Added small section about default values

Author: OskarStark

components/dotenv.rst

@@ -149,6 +149,13 @@ Use environment variables in values by prefixing variables with ``$``:
     its value will depend on the ``DB_USER`` value defined in other files
     instead of the value defined in this file.
 
+Define a default value in case the environment variable is not set:
+
+ .. code-block:: terminal
+
+    DB_USER=
+    DB_PASS=${DB_USER:-root}pass # results in DB_PASS=rootpass
+
 Embed commands via ``$()`` (not supported on Windows):
 
 .. code-block:: terminal