[TASK] Describe TYPO3 constant (#3597)

Releases: main, 12.4, 11.5

Co-authored-by: Chris Müller <2566282+brotkrueml@users.noreply.github.com>

Documentation/ApiOverview/GlobalValues/Constants/Index.rst

@@ -69,6 +69,38 @@ Available in frontend:
  Yes
 
 
+.. index::
+ Constants; TYPO3
+.. _globals-constants-typo3:
+
+TYPO3
+-----
+
+TYPO3 still has some extension PHP script files executed in global context
+without class or callable encapsulation, namely :file:`ext_localconf.php`,
+:file:`ext_tables.php` and files within :file:`Configuration/TCA/Overrides/`.
+When those files are located within the public document root of an instance and
+called via HTTP directly, they may error out and render error messages. This can
+be a security risk. To prevent this, those files **must** have a security gate
+as first line:
+
+.. code-block:: php
+
+ <?php
+
+ defined('TYPO3') or die();
+
+ // ... your code
+
+It is defined to :php:`true` in early TYPO3 bootstrap.
+
+.. seealso::
+
+ * :ref:`ext_localconf.php <ext-localconf-php>`
+ * :ref:`ext_tables.php <ext-tables-php>`
+ * :ref:`Configuration/TCA/Overrides/ <extension-configuration-tca-overrides>`
+
+
 .. index:: Constants; Filetypes
 
 File types

Documentation/ExtensionArchitecture/FileStructure/ExtLocalconf.rst

@@ -110,6 +110,7 @@ The skeleton of the :file:`ext_localconf.php` looks like this:
  // Add your code here
  })();
 
+Read :ref:`why the check for the TYPO3 constant is necessary <globals-constants-typo3>`.
 
 .. index:: Extension development; PageTSconfig
 

Documentation/ExtensionArchitecture/FileStructure/ExtTables.rst

@@ -122,6 +122,8 @@ new records of your table to be added on Standard pages call:
  'tx_myextension_domain_model_mymodel'
  );
 
+Read :ref:`why the check for the TYPO3 constant is necessary <globals-constants-typo3>`.
+
 .. index:: Extension development; Scheduler task registration
 .. _extension-configuration-files-scheduler:
 

Documentation/ExtensionArchitecture/HowTo/ExtendingTca/Examples/Index.rst

@@ -66,6 +66,8 @@ Here's the complete code, taken from file
  'tx_examples_options, tx_examples_special'
  );
 
+Read :ref:`why the check for the TYPO3 constant is necessary <globals-constants-typo3>`.
+
 .. note::
 
  The second example :php:`tx_examples_special` only works when the