diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 3b3f8334277e102c1331fdf7e8845923609ea26e..ed0679254ccec218b70646b46a5593b57a6ebb79 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -566,7 +566,11 @@ function variable_init($conf = array()) {
 }
 
 /**
- * Return a persistent variable.
+ * Returns a persistent variable.
+ *
+ * Case-sensitivity of the variable_* functions depends on the database
+ * collation used. To avoid problems, always use lower case for persistent
+ * variable names.
  *
  * @param $name
  *   The name of the variable to return.
@@ -584,7 +588,11 @@ function variable_get($name, $default) {
 }
 
 /**
- * Set a persistent variable.
+ * Sets a persistent variable.
+ *
+ * Case-sensitivity of the variable_* functions depends on the database
+ * collation used. To avoid problems, always use lower case for persistent
+ * variable names.
  *
  * @param $name
  *   The name of the variable to set.
@@ -609,7 +617,11 @@ function variable_set($name, $value) {
 }
 
 /**
- * Unset a persistent variable.
+ * Unsets a persistent variable.
+ *
+ * Case-sensitivity of the variable_* functions depends on the database
+ * collation used. To avoid problems, always use lower case for persistent
+ * variable names.
  *
  * @param $name
  *   The name of the variable to undefine.