Index: modules/simpletest/drupal_web_test_case.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/drupal_web_test_case.php,v
retrieving revision 1.125
diff -u -r1.125 drupal_web_test_case.php
--- modules/simpletest/drupal_web_test_case.php	11 Jul 2009 06:14:48 -0000	1.125
+++ modules/simpletest/drupal_web_test_case.php	14 Jul 2009 01:27:25 -0000
@@ -388,6 +388,7 @@
     }
 
     set_error_handler(array($this, 'errorHandler'));
+    drupal_register_debug_function('simpletest_drupal_debug');
     $methods = array();
     // Iterate through all the methods in this class.
     foreach (get_class_methods(get_class($this)) as $method) {
@@ -2490,3 +2491,17 @@
   }
   return FALSE;
 }
+
+function simpletest_drupal_debug($data, $label = NULL) {
+  // Print $data contents to string.
+  $string = print_r($data, TRUE) . "\n";
+  if ($label) {
+    $string = $label . ': ' . $string;
+  }
+
+  $file = file_directory_path() . '/drupal_debug.log';
+  if (file_put_contents($file, $string, FILE_APPEND) === FALSE) {
+    drupal_set_message(t('The temporary debug log cound not be written.'), 'error');
+    return FALSE;
+  }
+}
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 01:27:25 -0000
@@ -4794,3 +4794,41 @@
   }
   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 . ': ' . $string;
+  }
+
+  if (isset($_SERVER['HTTP_USER_AGENT']) && preg_match("/^simpletest\d+$/", $_SERVER['HTTP_USER_AGENT'])) {
+    $file = file_directory_path() . '/drupal_debug.log';
+  }
+  else {
+    $file = file_directory_temp() . '/drupal_debug.log';
+  }
+  if (file_put_contents($file, $string, FILE_APPEND) === FALSE) {
+    drupal_set_message(t('The temporary debug log cound not be written.'), 'error');
+    return FALSE;
+  }
+}