--- a/.gitmodules
+++ b/.gitmodules
@@ -7,4 +7,7 @@
 [submodule "lib/springy"]
 	path = lib/springy
 	url = https://github.com/dhotson/springy.git
+[submodule "lib/php-diff"]
+	path = lib/php-diff
+	url = https://github.com/chrisboulton/php-diff.git