Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.933
diff -u -r1.933 common.inc
--- includes/common.inc	13 Jul 2009 21:09:53 -0000	1.933
+++ includes/common.inc	14 Jul 2009 00:17:52 -0000
@@ -4794,3 +4794,37 @@
   }
   variable_set('css_js_query_string', $new_character . substr($string_history, 0, 19));
 }
+
+function drupal_register_debug_function($function = NULL) {
+  $debug_function = &drupal_static(__FUNCTION__, '_drupal_debug');
+
+  if ($function) {
+    $debug_function = $function;
+  }
+
+  return $debug_function;
+}
+
+function dd($data, $label = NULL) {
+  drupal_debug($data, $label);
+}
+
+function drupal_debug($data, $label = NULL) {
+  $function = drupal_register_debug_function();
+  $function($data, $label);
+}
+
+function _drupal_debug($data, $label = NULL) {
+  // Print $data contents to string.
+  $string = print_r($data, TRUE) . "\n";
+  if ($label) {
+    $string = $label . ': ' . $string;
+  }
+
+  // The temp directory does vary across multiple simpletest instances.
+  $file = file_directory_temp() . '/drupal_debug.log';
+  if (file_put_contents($file, $string, FILE_APPEND) === FALSE) {
+    drupal_set_message(t('The temporary debug log cound not be written.'), 'error');
+    return FALSE;
+  }
+}