diff -u b/core/includes/errors.inc b/core/includes/errors.inc
--- b/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -279,7 +279,7 @@
         // once more in the backtrace.
         array_shift($backtrace);
         // Generate a backtrace containing only scalar argument values.
-        $message .= '<pre class="backtrace">' . _drupal_format_backtrace($backtrace) . '</pre>';
+        $message .= '<pre class="backtrace">' . format_backtrace($backtrace) . '</pre>';
       }
       drupal_set_message($message, $class);
     }
@@ -342,13 +342,13 @@
  *
  * The calls show values for scalar arguments and type names for complex ones.
  *
- * @param $backtrace
- *   A standard PHP backtrace. Passed by reference.
+ * @param array $backtrace
+ *   A standard PHP backtrace.
  *
- * @return
+ * @return string
  *   A plain-text line-wrapped string ready to be put inside <pre>.
  */
-function _drupal_format_backtrace(array &$backtrace) {
+function format_backtrace(array $backtrace) {
   $return = '';
   foreach ($backtrace as $trace) {
     $call = array('function' => '', 'args' => array());