Add file option php_class_prefix (#2849)

This option will be prepended to generated classes of all messages in
the containing file.
diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index 4c3bca2..39e6c6c 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -9,6 +9,7 @@
 use Google\Protobuf\Internal\MapField;
 use Google\Protobuf\Internal\GPBType;
 use Foo\TestEnum;
+use Foo\TestIncludePrefixMessage;
 use Foo\TestMessage;
 use Foo\TestMessage_Sub;
 
@@ -838,4 +839,17 @@
     {
         $m = new NoNameSpaceEnum();
     }
+
+    #########################################################
+    # Test message with given prefix.
+    #########################################################
+
+    public function testPrefixMessage()
+    {
+        $m = new TestIncludePrefixMessage();
+        $n = new PrefixTestPrefix();
+        $n->setA(1);
+        $m->setPrefixMessage($n);
+        $this->assertSame(1, $m->getPrefixMessage()->getA());
+    }
 }