Fix field mask for php in json (#5591)

* Fix field mask for php in json

* Update byte size

php/tests/php_implementation_test.php

diff --git a/php/tests/php_implementation_test.php b/php/tests/php_implementation_test.php
index 323a36f..8d27d19 100644
--- a/php/tests/php_implementation_test.php
+++ b/php/tests/php_implementation_test.php
@@ -504,7 +504,7 @@
     {
         $m = new TestMessage();
         TestUtil::setTestMessage($m);
-        $this->assertSame(506, $m->byteSize());
+        $this->assertSame(518, $m->byteSize());
     }
 
     public function testPackedByteSize()