diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
index 10976b2..9ff22e1 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
@@ -199,7 +199,7 @@ protected function processField($field) {
     // Set the correct database-engine specific datatype.
     // In case one is already provided, force it to uppercase.
     if (isset($field['mysql_type'])) {
-      $field['mysql_type'] = drupal_strtoupper($field['mysql_type']);
+      $field['mysql_type'] = Unicode::strtoupper($field['mysql_type']);
     }
     else {
       $map = $this->getFieldTypeMap();
diff --git a/core/lib/Drupal/Core/Database/Driver/sqlite/Schema.php b/core/lib/Drupal/Core/Database/Driver/sqlite/Schema.php
index 669a722..cb836cd 100644
--- a/core/lib/Drupal/Core/Database/Driver/sqlite/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/sqlite/Schema.php
@@ -125,7 +125,7 @@ protected function processField($field) {
     // Set the correct database-engine specific datatype.
     // In case one is already provided, force it to uppercase.
     if (isset($field['sqlite_type'])) {
-      $field['sqlite_type'] = drupal_strtoupper($field['sqlite_type']);
+      $field['sqlite_type'] = Unicode::strtoupper($field['sqlite_type']);
     }
     else {
       $map = $this->getFieldTypeMap();
diff --git a/core/lib/Drupal/Core/Database/Query/Condition.php b/core/lib/Drupal/Core/Database/Query/Condition.php
index 0f2e1a6..4dc70f3 100644
--- a/core/lib/Drupal/Core/Database/Query/Condition.php
+++ b/core/lib/Drupal/Core/Database/Query/Condition.php
@@ -311,7 +311,7 @@ protected function mapConditionOperator($operator) {
     }
     else {
       // We need to upper case because PHP index matches are case sensitive but
-      // do not need the more expensive drupal_strtoupper because SQL statements are ASCII.
+      // do not need the more expensive Unicode::strtoupper() because SQL statements are ASCII.
       $operator = strtoupper($operator);
       $return = isset($specials[$operator]) ? $specials[$operator] : array();
     }
diff --git a/core/lib/Drupal/Core/Mail/MailFormatHelper.php b/core/lib/Drupal/Core/Mail/MailFormatHelper.php
index 3f180cc..707feb4 100644
--- a/core/lib/Drupal/Core/Mail/MailFormatHelper.php
+++ b/core/lib/Drupal/Core/Mail/MailFormatHelper.php
@@ -225,12 +225,12 @@ public static function htmlToText($string, $allowed_tags = NULL) {
           // Fancy headers.
           case 'h1':
             $indent[] = '======== ';
-            $casing = 'drupal_strtoupper';
+            $casing = '\Drupal\Component\Utility\Unicode::strtoupper';
             break;
 
           case 'h2':
             $indent[] = '-------- ';
-            $casing = 'drupal_strtoupper';
+            $casing = '\Drupal\Component\Utility\Unicode::strtoupper';
             break;
 
           case '/h1':
@@ -272,7 +272,7 @@ public static function htmlToText($string, $allowed_tags = NULL) {
       if (isset($chunk)) {
         // Apply any necessary case conversion.
         if (isset($casing)) {
-          $chunk = $casing($chunk);
+          $chunk = call_user_func($casing, $chunk);
         }
         $line_endings = Settings::get('mail_line_endings', PHP_EOL);
         // Format it and apply the current indentation.
diff --git a/core/modules/system/src/Tests/Mail/HtmlToTextTest.php b/core/modules/system/src/Tests/Mail/HtmlToTextTest.php
index e48ba42..0b8315c 100644
--- a/core/modules/system/src/Tests/Mail/HtmlToTextTest.php
+++ b/core/modules/system/src/Tests/Mail/HtmlToTextTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\system\Tests\Mail;
 
 use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Site\Settings;
 use Drupal\simpletest\WebTestBase;
 
@@ -242,8 +243,8 @@ public function testDrupalHtmlToTextBlockTagToNewline() {
     if (!$pass) {
       $this->verbose($this->stringToHtml($output));
     }
-    $output_upper = drupal_strtoupper($output);
-    $upper_input = drupal_strtoupper($input);
+    $output_upper = Unicode::strtoupper($output);
+    $upper_input = Unicode::strtoupper($input);
     $upper_output = drupal_html_to_text($upper_input);
     $pass = $this->assertEqual(
       $upper_output,