diff --git a/reference/outcontrol/functions/output-add-rewrite-var.xml b/reference/outcontrol/functions/output-add-rewrite-var.xml
index 59d0594b76..63bb40d3a7 100644
--- a/reference/outcontrol/functions/output-add-rewrite-var.xml
+++ b/reference/outcontrol/functions/output-add-rewrite-var.xml
@@ -26,6 +26,9 @@
     <link linkend="ini.url-rewriter.hosts">url_rewriter.hosts</link> &php.ini;
    parameters.
   </para>
+  <para>
+   Note that this function can be successfully called at most once per request.
+  </para>
   <note>
    <simpara>
     Calling this function will implicitly start output buffering if it is