Prepend "PB" to generated classes whose name are reserved words. (#2990)

diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index c5dee2d..554d2be 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -865,4 +865,15 @@
         $m->setPrefixMessage($n);
         $this->assertSame(1, $m->getPrefixMessage()->getA());
     }
+
+    #########################################################
+    # Test prefix for reserved words.
+    #########################################################
+
+    public function testPrefixForReservedWords()
+    {
+        $m = new \Foo\TestMessage_Empty();
+        $m = new \Foo\PBEmpty();
+        $m = new \PrefixEmpty();
+    }
 }