Add test for bug #79031

Fixed by preceding revert.

NEWS

@@ -47,6 +47,9 @@ PHP                                                                        NEWS
   . Fixed bug #78982 (pdo_pgsql returns dead persistent connection). (SATŌ
     Kentarō)
 
+- Session:
+  . Fixed bug #79031 (Session unserialization problem). (Nikita)
+
 - Spl:
   . Fixed bug #78976 (SplFileObject::fputcsv returns -1 on failure). (cmb)
 

ext/session/tests/bug79031.phpt

@@ -0,0 +1,71 @@
+--TEST--
+Bug #79031: Session unserialization problem
+--FILE--
+<?php
+
+class SerializableClass implements Serializable {
+    public $sharedProp;
+    public function __construct($prop)
+    {
+        $this->sharedProp = $prop;
+    }
+    public function __set($key, $value)
+    {
+        $this->$key = $value;
+    }
+    public function serialize()
+    {
+        return serialize(get_object_vars($this));
+    }
+    public function unserialize($data)
+    {
+        $ar = unserialize($data);
+        if ($ar === false) {
+            return;
+        }
+        foreach ($ar as $k => $v) {
+            $this->__set($k, $v);
+        }
+    }
+}
+
+// Shared object that acts as property of two another objects stored in session
+$testPropertyObj = new stdClass();
+$testPropertyObj->name = 'test';
+
+// Two instances of \SerializableClass that shares property
+$sessionObject = [
+    'obj1' => new SerializableClass($testPropertyObj),
+    'obj2' => new SerializableClass($testPropertyObj),
+];
+session_start();
+$_SESSION = $sessionObject;
+
+$sessionString = session_encode();
+session_decode($sessionString);
+echo $sessionString;
+echo "\n\n";
+var_dump($_SESSION);
+
+?>
+--EXPECT--
+obj1|C:17:"SerializableClass":65:{a:1:{s:10:"sharedProp";O:8:"stdClass":1:{s:4:"name";s:4:"test";}}}obj2|C:17:"SerializableClass":28:{a:1:{s:10:"sharedProp";r:3;}}
+
+array(2) {
+  ["obj1"]=>
+  object(SerializableClass)#4 (1) {
+    ["sharedProp"]=>
+    object(stdClass)#5 (1) {
+      ["name"]=>
+      string(4) "test"
+    }
+  }
+  ["obj2"]=>
+  object(SerializableClass)#6 (1) {
+    ["sharedProp"]=>
+    object(stdClass)#5 (1) {
+      ["name"]=>
+      string(4) "test"
+    }
+  }
+}