Fix the bug that message without namespace is not found in the descriptor pool. (#2240)

diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index 56466ca..d1a0bd5 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -1,6 +1,7 @@
 <?php
 
 require_once('test.pb.php');
+require_once('test_no_namespace.pb.php');
 require_once('test_util.php');
 
 use Google\Protobuf\Internal\RepeatedField;
@@ -554,4 +555,12 @@
         $this->assertSame('', $m->getOneofString());
         $this->assertSame(1, $m->getOneofMessage()->getA());
     }
+
+    #########################################################
+    # Test oneof field.
+    #########################################################
+
+    public function testMessageWithoutNamespace() {
+      $m = new NoNameSpace();
+    }
 }