diff --git a/ext/mbstring/tests/bug79154.phpt b/ext/mbstring/tests/bug79154.phpt
new file mode 100644
index 00000000000..dafac1bd5da
--- /dev/null
+++ b/ext/mbstring/tests/bug79154.phpt
@@ -0,0 +1,34 @@
+--TEST--
+Bug 79154 (mb_convert_encoding() can modify $from_encoding)
+--SKIPIF--
+<?php
+if (!extension_loaded('mbstring')) die('mbstring extension not available');
+?>
+--FILE--
+<?php
+class Utf8Encoding
+{
+    public function __toString()
+    {
+        return 'UTF-8';
+    }
+}
+
+$utf8encoding = new Utf8Encoding();
+$encodings = [$utf8encoding];
+var_dump($encodings);
+mb_convert_encoding('foo', 'UTF-8', $encodings);
+var_dump($encodings);
+
+?>
+--EXPECTF--
+array(1) {
+  [0]=>
+  object(Utf8Encoding)#%d (0) {
+  }
+}
+array(1) {
+  [0]=>
+  object(Utf8Encoding)#%d (0) {
+  }
+}