diff --git a/reference/outcontrol/functions/ob-start.xml b/reference/outcontrol/functions/ob-start.xml
index 7a16b40f8d..c21dde467c 100644
--- a/reference/outcontrol/functions/ob-start.xml
+++ b/reference/outcontrol/functions/ob-start.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.16 $ -->
+<!-- $Revision: 1.17 $ -->
 <!-- splitted from ./en/functions/outcontrol.xml, last change in rev 1.1 -->
   <refentry id="function.ob-start">
    <refnamediv>
@@ -59,6 +59,14 @@
       <parameter>output_callback</parameter> can not be executed.
      </para>
     </note>
+    <warning>
+     <para>
+      Some web servers (e.g. Apache) change the working directory of a script
+      when calling the callback function. You can change it back by e.g.
+      <literal>chdir(dirname($_SERVER['PHP_SELF']))</literal> in the callback
+      function.
+     </para>
+    </warning>
     <para>
      If the optional parameter <parameter>chunk_size</parameter> is passed, the
      callback function is called on every first newline after