Updates core (PHP warning in debug mode)

diff --git a/core b/core
index b08bfbc..7f8e81b 160000 (submodule)
--- a/core
+++ b/core
@@ -1 +1 @@
-Subproject commit b08bfbce605c3632f240794556a6a0828c8bf153
+Subproject commit 7f8e81bbf66f2fe959e93f865fd74a08e20e9fee