diff --git a/data/helpers.d/php b/data/helpers.d/php
index d5b17c58f..747d40321 100644
--- a/data/helpers.d/php
+++ b/data/helpers.d/php
@@ -1,6 +1,6 @@
 #!/bin/bash
 
-YNH_DEFAULT_PHP_VERSION=7.0
+readonly YNH_DEFAULT_PHP_VERSION=7.0
 # Declare the actual php version to use.
 # A packager willing to use another version of php can override the variable into its _common.sh.
 YNH_PHP_VERSION=${YNH_PHP_VERSION:-$YNH_DEFAULT_PHP_VERSION}