diff --git a/reference/array/functions/array-walk.xml b/reference/array/functions/array-walk.xml
index e1a15bdef9..fed4769414 100644
--- a/reference/array/functions/array-walk.xml
+++ b/reference/array/functions/array-walk.xml
@@ -53,6 +53,14 @@
         original array itself.
        </para>
       </note>
+      <note>
+        <para>
+          Many internal functions (for example <function>strtolower</function>)
+          will throw a warning if more than the expected number of argument
+          are passed in and are not usable directly as
+          <parameter>funcname</parameter>.
+        </para>
+      </note>
       <para>
        Only the values of the <parameter>array</parameter> may potentially be
        changed; its structure cannot be altered, i.e., the programmer cannot
diff --git a/reference/filesystem/functions/popen.xml b/reference/filesystem/functions/popen.xml
index c4f89faf47..0d58563f71 100644
--- a/reference/filesystem/functions/popen.xml
+++ b/reference/filesystem/functions/popen.xml
@@ -51,7 +51,10 @@
    only be used for reading or writing) and must be closed with
    <function>pclose</function>. This pointer may be used with
    <function>fgets</function>, <function>fgetss</function>, and
-   <function>fwrite</function>.
+   <function>fwrite</function>. When the mode is 'r', the returned
+   file pointer equals to the STDOUT of the command, when the mode
+   is 'w', the returned file pointer equals to the STDIN of the
+   command.
   </para>
   <para>
    If an error occurs, returns &false;.