diff --git includes/file.inc includes/file.inc
index ff0d066..b59d607 100644
--- includes/file.inc
+++ includes/file.inc
@@ -2194,11 +2194,11 @@ function drupal_dirname($uri) {
  * @ingroup php_wrappers
  */
 function drupal_mkdir($uri, $mode = NULL, $recursive = FALSE, $context = NULL) {
-  if (is_null($mode)) {
+  if (!isset($mode)) {
     $mode = variable_get('file_chmod_directory', 0775);
   }
 
-  if (is_null($context)) {
+  if (!isset($context)) {
     return mkdir($uri, $mode, $recursive);
   }
   else {
@@ -2283,7 +2283,7 @@ function drupal_tempnam($directory, $prefix) {
 function file_directory_temp() {
   $temporary_directory = variable_get('file_directory_temp', NULL);
 
-  if (is_null($temporary_directory)) {
+  if (!isset($temporary_directory)) {
     $directories = array();
 
     // Has PHP been set with an upload_tmp_dir?
diff --git includes/form.inc includes/form.inc
index a0a9b00..2b55c07 100644
--- includes/form.inc
+++ includes/form.inc
@@ -2062,7 +2062,7 @@ function form_type_checkboxes_value($element, $input = FALSE) {
     // checkboxes to the server at all). This will not affect non-programmatic
     // form submissions, since a checkbox can never legitimately be NULL.
     foreach ($input as $key => $value) {
-      if (is_null($value)) {
+      if (!isset($value)) {
         unset($input[$key]);
       }
     }
@@ -3887,7 +3887,7 @@ function _batch_queue($batch_set) {
 
   // The class autoloader is not available when running update.php, so make
   // sure the files are manually included.
-  if (is_null($queues)) {
+  if (!isset($queues)) {
     $queues = array();
     require_once DRUPAL_ROOT . '/modules/system/system.queue.inc';
     require_once DRUPAL_ROOT . '/includes/batch.queue.inc';
diff --git includes/theme.inc includes/theme.inc
index eeefc74..3a8e02b 100644
--- includes/theme.inc
+++ includes/theme.inc
@@ -1118,7 +1118,7 @@ function theme_get_setting($setting_name, $theme = NULL) {
   $cache = &drupal_static(__FUNCTION__, array());
 
   // If no key is given, use the current theme if we can determine it.
-  if (is_null($theme)) {
+  if (!isset($theme)) {
     $theme = !empty($GLOBALS['theme_key']) ? $GLOBALS['theme_key'] : '';
   }
 
diff --git scripts/run-tests.sh scripts/run-tests.sh
index 748fbbe..bb7a244 100755
--- scripts/run-tests.sh
+++ scripts/run-tests.sh
@@ -302,7 +302,7 @@ function simpletest_script_init($server_software) {
 function simpletest_script_execute_batch() {
   global $args;
 
-  if (is_null($args['test-id'])) {
+  if (!isset($args['test-id'])) {
     simpletest_script_print_error("--execute-batch should not be called interactively.");
     exit;
   }