diff --git a/docs/guide/helper-array.md b/docs/guide/helper-array.md
index 19c3c03ad5f..e7b1641fff8 100644
--- a/docs/guide/helper-array.md
+++ b/docs/guide/helper-array.md
@@ -483,3 +483,140 @@ ArrayHelper::isIn('a', new ArrayObject(['a']));
 // true 
 ArrayHelper::isSubset(new ArrayObject(['a', 'c']), new ArrayObject(['a', 'b', 'c']));
 ```
+
+## Flattening Arrays <span id="flattening-arrays"></span>
+
+The `ArrayHelper::flatten()` method allows you to convert a multi-dimensional array into a single-dimensional array by concatenating keys.
+
+### Basic Usage
+
+To flatten a nested array, simply pass the array to the `flatten()` method:
+
+```php
+$array = [
+    'a' => [
+        'b' => [
+            'c' => 1,
+            'd' => 2,
+        ],
+        'e' => 3,
+    ],
+    'f' => 4,
+];
+
+$flattenedArray = ArrayHelper::flatten($array);
+// Result:
+// [
+//     'a.b.c' => 1,
+//     'a.b.d' => 2,
+//     'a.e' => 3,
+//     'f' => 4,
+// ]
+```
+
+### Custom Separator
+
+You can specify a custom separator to use when concatenating keys:
+
+```php
+$array = [
+    'a' => [
+        'b' => [
+            'c' => 1,
+            'd' => 2,
+        ],
+        'e' => 3,
+    ],
+    'f' => 4,
+];
+
+$flattenedArray = ArrayHelper::flatten($array, '_');
+// Result:
+// [
+//     'a_b_c' => 1,
+//     'a_b_d' => 2,
+//     'a_e' => 3,
+//     'f' => 4,
+// ]
+```
+
+### Handling Special Characters in Keys
+
+The `flatten()` method can handle keys with special characters:
+
+```php
+$array = [
+    'a.b' => [
+        'c.d' => 1,
+    ],
+    'e.f' => 2,
+];
+
+$flattenedArray = ArrayHelper::flatten($array);
+// Result:
+// [
+//     'a.b.c.d' => 1,
+//     'e.f' => 2,
+// ]
+```
+
+### Mixed Data Types
+
+The `flatten()` method works with arrays containing different data types:
+
+```php
+$array = [
+    'a' => [
+        'b' => 'string',
+        'c' => 123,
+        'd' => true,
+        'e' => null,
+    ],
+    'f' => [1, 2, 3],
+];
+
+$flattenedArray = ArrayHelper::flatten($array);
+// Result:
+// [
+//     'a.b' => 'string',
+//     'a.c' => 123,
+//     'a.d' => true,
+//     'a.e' => null,
+//     'f.0' => 1,
+//     'f.1' => 2,
+//     'f.2' => 3,
+// ]
+```
+
+### Edge Cases
+
+The `flatten()` method handles various edge cases, such as empty arrays and non-array values:
+
+```php
+// Empty array
+$array = [];
+$flattenedArray = ArrayHelper::flatten($array);
+// Result: []
+
+// Non-array value
+$array = 'string';
+$flattenedArray = ArrayHelper::flatten($array);
+// Result:
+// yii\base\InvalidArgumentException: Argument $array must be an array or implement Traversable
+```
+
+### Key Collisions
+
+When keys collide, the `flatten()` method will overwrite the previous value:
+
+```php
+$array = [
+    'a' => [
+        'b' => 1,
+    ],
+    'a.b' => 2,
+];
+
+$flattenedArray = ArrayHelper::flatten($array);
+// Result: ['a.b' => 2]
+```