diff --git a/docs/en/designers/language-modifiers/language-modifier-lower.xml b/docs/en/designers/language-modifiers/language-modifier-lower.xml
index 13c35bce..6577370a 100644
--- a/docs/en/designers/language-modifiers/language-modifier-lower.xml
+++ b/docs/en/designers/language-modifiers/language-modifier-lower.xml
@@ -3,7 +3,8 @@
  <sect1 id="language.modifier.lower">
   <title>lower</title>
   <para>
-   This is used to lowercase a variable.
+   This is used to lowercase a variable.This is equivalent to the PHP
+   <ulink url="&url.php-manual;strtolower">strtolower()</ulink> function.
   </para>
   <example>
    <title>lower</title>
diff --git a/docs/en/designers/language-modifiers/language-modifier-upper.xml b/docs/en/designers/language-modifiers/language-modifier-upper.xml
index 2cf23282..d985f87d 100644
--- a/docs/en/designers/language-modifiers/language-modifier-upper.xml
+++ b/docs/en/designers/language-modifiers/language-modifier-upper.xml
@@ -3,7 +3,8 @@
  <sect1 id="language.modifier.upper">
   <title>upper</title>
   <para>
-   This is used to uppercase a variable.
+   This is used to uppercase a variable. This is equivalent to the PHP
+   <ulink url="&url.php-manual;strtoupper">strtoupper()</ulink> function.
   </para>
   <example>
    <title>upper</title>