diff --git a/composer.json b/composer.json
index c264617..85e2e5a 100644
--- a/composer.json
+++ b/composer.json
@@ -1,6 +1,7 @@
 {
   "require" : {
-        "ext-mbstring": "*"
+    "php": "^5.6.0",
+    "ext-mbstring": "*"
   },
   "require-dev": {
     "heroku/heroku-buildpack-php": "*"