diff --git a/overlay/etc/templates/constants.local.php.tmpl b/overlay/etc/templates/constants.local.php.tmpl
index c11cb21..ac25c40 100644
--- a/overlay/etc/templates/constants.local.php.tmpl
+++ b/overlay/etc/templates/constants.local.php.tmpl
@@ -1,4 +1,10 @@
 <?php
+function safe_define($name, $value) {
+    if (!defined($name)) {
+        return define($name, $value);
+    }
+}
+
 safe_define('FRESHRSS_USERAGENT', 'FreshRSS/' . FRESHRSS_VERSION . ' (' . PHP_OS . '; ' . FRESHRSS_WEBSITE . ')');
 safe_define('PHP_COMPRESSION', false);
 safe_define('COPY_SYSLOG_TO_STDERR', true);