diff --git a/stubs/CoreGenericFunctions.phpstub b/stubs/CoreGenericFunctions.phpstub
index b3b00a169..260464756 100644
--- a/stubs/CoreGenericFunctions.phpstub
+++ b/stubs/CoreGenericFunctions.phpstub
@@ -1300,4 +1300,11 @@ function get_defined_constants(bool $categorize = false): array {}
  * @param class-string $class
  * @return ($allow_string is false ? ($object_or_class is object ? bool : false) : bool)
  */
-function is_a($object_or_class, string $class, $allow_string = false): bool{}
\ No newline at end of file
+function is_a($object_or_class, string $class, $allow_string = false): bool{}
+
+/**
+ * @template T of array|string
+ * @param T $string
+ * @return (T is array<int, string> ? array<int, string> : T is array ? array : string)
+ */
+function mb_convert_encoding(array|string $string, string $to_encoding, array|string|null $from_encoding = null): array|string|false{}