Require at least PHP 5.1.6.
diff --git a/composer.json b/composer.json
index b48f8c1..bcab60f 100644
--- a/composer.json
+++ b/composer.json
@@ -1,6 +1,7 @@
 {
     "name" : "ellislab/codeigniter",
     "require": {
+        "php": ">=5.1.6",
         "mikey179/vfsStream": "*"
     },
     "require-dev": {