minor #19914 [PropertyAccess] implement __isset() together with `__…

…get()` (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[PropertyAccess] implement `__isset()` together with `__get()`

see symfony/symfony#54739 (comment)

Commits
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b38c05e implement __isset() together with __get()

components/property_access.rst

@@ -204,12 +204,22 @@ The ``getValue()`` method can also use the magic ``__get()`` method::
         {
             return $this->children[$id];
         }
+
+        public function __isset($id): bool
+        {
+            return true;
+        }
     }
 
     $person = new Person();
 
     var_dump($propertyAccessor->getValue($person, 'Wouter')); // [...]
 
+.. caution::
+
+    When implementing the magic ``__get()`` method, you also need to implement
+    ``__isset()``.
+
 .. versionadded:: 5.2
 
     The magic ``__get()`` method can be disabled since in Symfony 5.2.