Merge branch 'PHP-5.6' into PHP-7.0

* PHP-5.6:
  backport to 5.6 (we should not unset the default value)

Conflicts:
	Zend/zend_exceptions.c
	ext/standard/tests/serialize/bug69152.phpt

ext/standard/tests/serialize/bug69152.phpt

@@ -9,7 +9,6 @@ $x->test();
 
 ?>
 --EXPECTF--
-Notice: Undefined property: Exception::$previous in %s on line %d
 Exception in %s:%d
 Stack trace:
 #0 {main}

ext/standard/tests/serialize/bug69793.phpt

@@ -7,8 +7,6 @@ $e = unserialize('O:9:"Exception":7:{s:17:"'."\0".'Exception'."\0".'string";s:1:
 var_dump($e."");
 ?>
 --EXPECTF--
-Notice: Undefined property: Exception::$message in %s%ebug69793.php on line %d
-
 Notice: Undefined property: Exception::$file in %s%ebug69793.php on line %d
 
 Notice: Undefined property: Exception::$previous in %s%ebug69793.php on line %d

ext/standard/tests/serialize/bug70963.phpt

@@ -8,7 +8,7 @@ var_dump(unserialize('a:2:{i:0;O:9:"exception":1:{s:16:"'."\0".'Exception'."\0".
 --EXPECTF--
 array(1) {
   [0]=>
-  object(Exception)#%d (5) {
+  object(Exception)#%d (6) {
     ["message":protected]=>
     string(0) ""
     ["string":"Exception":private]=>
@@ -18,12 +18,14 @@ array(1) {
     ["file":protected]=>
     string(%d) "%s"
     ["line":protected]=>
-    int(%d)
+    int(2)
+    ["previous":"Exception":private]=>
+    NULL
   }
 }
 array(1) {
   [0]=>
-  object(Exception)#%d (5) {
+  object(Exception)#%d (6) {
     ["message":protected]=>
     string(0) ""
     ["string":"Exception":private]=>
@@ -33,6 +35,8 @@ array(1) {
     ["file":protected]=>
     string(%d) "%s"
     ["line":protected]=>
-    int(%d)
+    int(3)
+    ["previous":"Exception":private]=>
+    NULL
   }
 }