diff --git includes/file.inc includes/file.inc
index 9ce8534..3f202a4 100644
--- includes/file.inc
+++ includes/file.inc
@@ -2193,11 +2193,11 @@ 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, $recursive);
   }
   else {
@@ -2282,7 +2282,7 @@ function drupal_tempnam($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 f171c67..37d2c83 100644
--- includes/form.inc
+++ includes/form.inc
@@ -2096,7 +2096,7 @@ 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[$key]);
       }
     }
@@ -4009,7 +4009,7 @@ function _batch_queue($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)) {
     $queues = array();
     require_once DRUPAL_ROOT . '/modules/system/system.queue.inc';
     require_once DRUPAL_ROOT . '/includes/batch.queue.inc';
diff --git includes/theme.inc includes/theme.inc
index cb6f58d..7a89e30 100644
--- includes/theme.inc
+++ includes/theme.inc
@@ -1119,7 +1119,7 @@ function theme_get_setting($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 modules/field/modules/field_sql_storage/field_sql_storage.module modules/field/modules/field_sql_storage/field_sql_storage.module
index 1971ef2..69311dd 100644
--- modules/field/modules/field_sql_storage/field_sql_storage.module
+++ modules/field/modules/field_sql_storage/field_sql_storage.module
@@ -106,7 +106,7 @@ function _field_sql_storage_indexname($name, $index) {
  */
 function _field_sql_storage_etid($entity_type) {
   $etid = variable_get('field_sql_storage_' . $entity_type . '_etid', NULL);
-  if (is_null($etid)) {
+  if (!isset($etid)) {
     $etid = db_insert('field_config_entity_type')->fields(array('type' => $entity_type))->execute();
     variable_set('field_sql_storage_' . $entity_type . '_etid', $etid);
   }
diff --git modules/field/tests/field_test.module modules/field/tests/field_test.module
index 7b91b94..4d8868b 100644
--- modules/field/tests/field_test.module
+++ modules/field/tests/field_test.module
@@ -152,7 +152,7 @@ function field_test_field_language_alter(&$display_language, $context) {
 function field_test_memorize($key = NULL, $value = NULL) {
   $memorize = &drupal_static(__FUNCTION__, NULL);
 
-  if (is_null($key)) {
+  if (!isset($key)) {
     $return = $memorize;
     $memorize = array();
     return $return;
diff --git modules/field/tests/field_test.storage.inc modules/field/tests/field_test.storage.inc
index 7ef9210..d5605cc 100644
--- modules/field/tests/field_test.storage.inc
+++ modules/field/tests/field_test.storage.inc
@@ -70,7 +70,7 @@ function field_test_field_storage_details_alter(&$details, $field) {
  * Helper function: stores or retrieves data from the 'storage backend'.
  */
 function _field_test_storage_data($data = NULL) {
-  if (is_null($data)) {
+  if (!isset($data)) {
     return variable_get('field_test_storage_data', array());
   }
   else {
@@ -318,7 +318,7 @@ function field_test_field_storage_query($field_id, $conditions, $count, &$cursor
       }
 
       if ($match) {
-        if (is_null($skip) || $skipped >= $skip) {
+        if (!isset($skip) || $skipped >= $skip) {
           $cursor++;
           // If querying all revisions and the entity type has revisions, we need
           // to key the results by revision_ids.
diff --git modules/field_ui/field_ui.admin.inc modules/field_ui/field_ui.admin.inc
index c3369fd..b790294 100644
--- modules/field_ui/field_ui.admin.inc
+++ modules/field_ui/field_ui.admin.inc
@@ -68,7 +68,7 @@ function field_ui_inactive_message($entity_type, $bundle) {
  * This is intended as a callback for array_reduce().
  */
 function _field_ui_reduce_order($array, $a) {
-  $array = is_null($array) ? array() : $array;
+  $array = !isset($array) ? array() : $array;
   if ($a['name']) {
     $array[] = $a['name'];
   }
diff --git modules/search/search.module modules/search/search.module
index c676876..7413e84 100644
--- modules/search/search.module
+++ modules/search/search.module
@@ -959,7 +959,7 @@ function search_form($form, &$form_state, $action = '', $keys = '', $type = NULL
   if (!$action) {
     $action = 'search/' . $module_info['path'];
   }
-  if (is_null($prompt)) {
+  if (!isset($prompt)) {
     $prompt = t('Enter your keywords');
   }
 
diff --git modules/simpletest/tests/batch_test.module modules/simpletest/tests/batch_test.module
index 14e7931..cfbb288 100644
--- modules/simpletest/tests/batch_test.module
+++ modules/simpletest/tests/batch_test.module
@@ -466,7 +466,7 @@ function batch_test_stack($data = NULL, $reset = FALSE) {
   if ($reset) {
     variable_del('batch_test_stack');
   }
-  if (is_null($data)) {
+  if (!isset($data)) {
     return variable_get('batch_test_stack', array());
   }
   $stack = variable_get('batch_test_stack', array());
diff --git modules/simpletest/tests/file.test modules/simpletest/tests/file.test
index 05d4e4b..863833c 100644
--- modules/simpletest/tests/file.test
+++ modules/simpletest/tests/file.test
@@ -174,7 +174,7 @@ class FileTestCase extends DrupalWebTestCase {
    */
   function createDirectory($path = NULL) {
     // A directory to operate on.
-    if (is_null($path)) {
+    if (!isset($path)) {
       $path = file_default_scheme() . '://' . $this->randomName();
     }
     $this->assertTrue(drupal_mkdir($path) && is_dir($path), t('Directory was created successfully.'));
@@ -198,12 +198,12 @@ class FileTestCase extends DrupalWebTestCase {
    *   File object.
    */
   function createFile($filepath = NULL, $contents = NULL, $scheme = 'public') {
-    if (is_null($filepath)) {
+    if (!isset($filepath)) {
       $filepath = $this->randomName();
     }
     $filepath = $scheme . '://' . $filepath;
 
-    if (is_null($contents)) {
+    if (!isset($contents)) {
       $contents = "file_put_contents() doesn't seem to appreciate empty strings so let's put in some data.";
     }
 
@@ -282,7 +282,7 @@ class FileHookTestCase extends FileTestCase {
   function assertFileHookCalled($hook, $expected_count = 1, $message = NULL) {
     $actual_count = count(file_test_get_calls($hook));
 
-    if (is_null($message)) {
+    if (!isset($message)) {
       if ($actual_count == $expected_count) {
         $message = t('hook_file_@name was called correctly.', array('@name' => $hook));
       }
diff --git modules/syslog/syslog.module modules/syslog/syslog.module
index d0a48df..e2a8bab 100644
--- modules/syslog/syslog.module
+++ modules/syslog/syslog.module
@@ -103,7 +103,7 @@ function syslog_watchdog(array $log_entry) {
     '!referer'     => $log_entry['referer'],
     '!uid'         => $log_entry['user']->uid,
     '!link'        => strip_tags($log_entry['link']),
-    '!message'     => strip_tags(is_null($log_entry['variables']) ? $log_entry['message'] : strtr($log_entry['message'], $log_entry['variables'])),
+    '!message'     => strip_tags(!isset($log_entry['variables']) ? $log_entry['message'] : strtr($log_entry['message'], $log_entry['variables'])),
   ));
 
   syslog($log_entry['severity'], $message);
diff --git modules/taxonomy/taxonomy.module modules/taxonomy/taxonomy.module
index 853184d..b36f336 100644
--- modules/taxonomy/taxonomy.module
+++ modules/taxonomy/taxonomy.module
@@ -841,7 +841,7 @@ function taxonomy_get_tree($vid, $parent = 0, $max_depth = NULL, $depth = -1) {
     }
   }
 
-  $max_depth = (is_null($max_depth)) ? count($children[$vid]) : $max_depth;
+  $max_depth = (!isset($max_depth)) ? count($children[$vid]) : $max_depth;
   $tree = array();
   if ($max_depth > $depth && !empty($children[$vid][$parent])) {
     foreach ($children[$vid][$parent] as $child) {
@@ -1038,7 +1038,7 @@ function taxonomy_implode_tags($tags, $vid = NULL) {
   $typed_tags = array();
   foreach ($tags as $tag) {
     // Extract terms belonging to the vocabulary in question.
-    if (is_null($vid) || $tag->vid == $vid) {
+    if (!isset($vid) || $tag->vid == $vid) {
       // Make sure we have a completed loaded taxonomy term.
       if (isset($tag->name)) {
         // Commas and quotes in tag names are special cases, so encode 'em.
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;
   }