diff --git a/CHANGELOG.md b/CHANGELOG.md
index f9d10a750..8d7ef8199 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix Variable Usage in Exception message when unable to load subtemplate [#808](https://github.com/smarty-php/smarty/pull/808)
 - Fixed PHP8.1 deprecation notices for strftime [#672](https://github.com/smarty-php/smarty/issues/672)
 - Fixed PHP8.1 deprecation errors passing null to parameter in trim [#807](https://github.com/smarty-php/smarty/pull/807)
+- Adapt Smarty upper/lower functions to be codesafe (e.g. for Turkish locale) [#586](https://github.com/smarty-php/smarty/pull/586)
 
 ## [4.2.1] - 2022-09-14
 
diff --git a/libs/Autoloader.php b/libs/Autoloader.php
index 1673ce2ff..c20814031 100644
--- a/libs/Autoloader.php
+++ b/libs/Autoloader.php
@@ -5,6 +5,11 @@
  * @package Smarty
  */
 
+
+if (!defined('SMARTY_HELPER_FUNCTIONS_LOADED')) {
+	include dirname(__FILE__) . '/functions.php';
+}
+
 /**
  * Smarty Autoloader
  *
@@ -89,7 +94,7 @@ public static function autoload($class)
         if ($class[ 0 ] !== 'S' || strpos($class, 'Smarty') !== 0) {
             return;
         }
-        $_class = strtolower($class);
+        $_class = smarty_strtolower_ascii($class);
         if (isset(self::$rootClasses[ $_class ])) {
             $file = self::$SMARTY_DIR . self::$rootClasses[ $_class ];
             if (is_file($file)) {
diff --git a/libs/Smarty.class.php b/libs/Smarty.class.php
index 21beafdf0..5507ebcde 100644
--- a/libs/Smarty.class.php
+++ b/libs/Smarty.class.php
@@ -60,12 +60,21 @@
      */
     define('SMARTY_MBSTRING', function_exists('mb_get_info'));
 }
+
+/**
+ * Load helper functions
+ */
+if (!defined('SMARTY_HELPER_FUNCTIONS_LOADED')) {
+    include dirname(__FILE__) . '/functions.php';
+}
+
 /**
  * Load Smarty_Autoloader
  */
 if (!class_exists('Smarty_Autoloader')) {
     include dirname(__FILE__) . '/bootstrap.php';
 }
+
 /**
  * Load always needed external class files
  */
diff --git a/libs/functions.php b/libs/functions.php
new file mode 100644
index 000000000..bac00e521
--- /dev/null
+++ b/libs/functions.php
@@ -0,0 +1,51 @@
+<?php
+/**
+ * This file is part of the Smarty package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+/**
+ * Registers some helper/polyfill functions.
+ */
+
+const SMARTY_HELPER_FUNCTIONS_LOADED = true;
+
+/**
+ * Converts the first characters in $string to uppercase (A-Z) if it is an ASCII lowercase character (a-z).
+ *
+ * May not be required when running PHP8.2+: https://wiki.php.net/rfc/strtolower-ascii
+ *
+ * @param $string
+ *
+ * @return string
+ */
+function smarty_ucfirst_ascii($string): string {
+    return smarty_strtoupper_ascii(substr($string, 0, 1)) . substr($string, 1);
+}
+
+/**
+ * Converts all uppercase ASCII characters (A-Z) in $string to lowercase (a-z).
+ *
+ * May not be required when running PHP8.2+: https://wiki.php.net/rfc/strtolower-ascii
+ *
+ * @param $string
+ *
+ * @return string
+ */
+function smarty_strtolower_ascii($string): string {
+    return strtr($string, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz');
+}
+
+/**
+ * Converts all lowercase ASCII characters (a-z) in $string to uppercase (A-Z).
+ *
+ * May not be required when running PHP8.2+: https://wiki.php.net/rfc/strtolower-ascii
+ *
+ * @param $string
+ *
+ * @return string
+ */
+function smarty_strtoupper_ascii($string): string {
+    return strtr($string, 'abcdefghijklmnopqrstuvwxyz', 'ABCDEFGHIJKLMNOPQRSTUVWXYZ');
+}
\ No newline at end of file
diff --git a/libs/sysplugins/smarty_cacheresource.php b/libs/sysplugins/smarty_cacheresource.php
index 91e9f3924..db68f9bfd 100644
--- a/libs/sysplugins/smarty_cacheresource.php
+++ b/libs/sysplugins/smarty_cacheresource.php
@@ -205,11 +205,11 @@ public static function load(Smarty $smarty, $type = null)
         }
         // try sysplugins dir
         if (isset(self::$sysplugins[ $type ])) {
-            $cache_resource_class = 'Smarty_Internal_CacheResource_' . ucfirst($type);
+            $cache_resource_class = 'Smarty_Internal_CacheResource_' . smarty_ucfirst_ascii($type);
             return $smarty->_cache[ 'cacheresource_handlers' ][ $type ] = new $cache_resource_class();
         }
         // try plugins dir
-        $cache_resource_class = 'Smarty_CacheResource_' . ucfirst($type);
+        $cache_resource_class = 'Smarty_CacheResource_' . smarty_ucfirst_ascii($type);
         if ($smarty->loadPlugin($cache_resource_class)) {
             return $smarty->_cache[ 'cacheresource_handlers' ][ $type ] = new $cache_resource_class();
         }
diff --git a/libs/sysplugins/smarty_internal_compile_private_foreachsection.php b/libs/sysplugins/smarty_internal_compile_private_foreachsection.php
index d3aab24bb..246350dc8 100644
--- a/libs/sysplugins/smarty_internal_compile_private_foreachsection.php
+++ b/libs/sysplugins/smarty_internal_compile_private_foreachsection.php
@@ -143,7 +143,7 @@ public function matchProperty($source)
         foreach ($this->resultOffsets as $key => $offset) {
             foreach ($match[ $offset ] as $m) {
                 if (!empty($m)) {
-                    $this->matchResults[ $key ][ strtolower($m) ] = true;
+                    $this->matchResults[ $key ][ smarty_strtolower_ascii($m) ] = true;
                 }
             }
         }
@@ -213,12 +213,12 @@ public function matchBlockSource(Smarty_Internal_TemplateCompilerBase $compiler)
      */
     public function compileSpecialVariable($args, Smarty_Internal_TemplateCompilerBase $compiler, $parameter)
     {
-        $tag = strtolower(trim($parameter[ 0 ], '"\''));
+        $tag = smarty_strtolower_ascii(trim($parameter[ 0 ], '"\''));
         $name = isset($parameter[ 1 ]) ? $compiler->getId($parameter[ 1 ]) : false;
         if (!$name) {
             $compiler->trigger_template_error("missing or illegal \$smarty.{$tag} name attribute", null, true);
         }
-        $property = isset($parameter[ 2 ]) ? strtolower($compiler->getId($parameter[ 2 ])) : false;
+        $property = isset($parameter[ 2 ]) ? smarty_strtolower_ascii($compiler->getId($parameter[ 2 ])) : false;
         if (!$property || !in_array($property, $this->nameProperties)) {
             $compiler->trigger_template_error("missing or illegal \$smarty.{$tag} property attribute", null, true);
         }
diff --git a/libs/sysplugins/smarty_internal_compile_private_special_variable.php b/libs/sysplugins/smarty_internal_compile_private_special_variable.php
index d53ef51ff..590cba5af 100644
--- a/libs/sysplugins/smarty_internal_compile_private_special_variable.php
+++ b/libs/sysplugins/smarty_internal_compile_private_special_variable.php
@@ -29,7 +29,7 @@ class Smarty_Internal_Compile_Private_Special_Variable extends Smarty_Internal_C
     public function compile($args, Smarty_Internal_TemplateCompilerBase $compiler, $parameter)
     {
         $_index = preg_split("/\]\[/", substr($parameter, 1, strlen($parameter) - 2));
-        $variable = strtolower($compiler->getId($_index[ 0 ]));
+        $variable = smarty_strtolower_ascii($compiler->getId($_index[ 0 ]));
         if ($variable === false) {
             $compiler->trigger_template_error("special \$Smarty variable name index can not be variable", null, true);
         }
@@ -40,7 +40,7 @@ public function compile($args, Smarty_Internal_TemplateCompilerBase $compiler, $
                 case 'foreach':
                 case 'section':
                     if (!isset(Smarty_Internal_TemplateCompilerBase::$_tag_objects[ $variable ])) {
-                        $class = 'Smarty_Internal_Compile_' . ucfirst($variable);
+                        $class = 'Smarty_Internal_Compile_' . smarty_ucfirst_ascii($variable);
                         Smarty_Internal_TemplateCompilerBase::$_tag_objects[ $variable ] = new $class;
                     }
                     return Smarty_Internal_TemplateCompilerBase::$_tag_objects[ $variable ]->compileSpecialVariable(
@@ -76,7 +76,7 @@ public function compile($args, Smarty_Internal_TemplateCompilerBase $compiler, $
                         $compiler->trigger_template_error("(secure mode) super globals not permitted");
                         break;
                     }
-                    $compiled_ref = '$_' . strtoupper($variable);
+                    $compiled_ref = '$_' . smarty_strtoupper_ascii($variable);
                     break;
                 case 'template':
                     return 'basename($_smarty_tpl->source->filepath)';
diff --git a/libs/sysplugins/smarty_internal_extension_handler.php b/libs/sysplugins/smarty_internal_extension_handler.php
index b07615526..634ad8315 100644
--- a/libs/sysplugins/smarty_internal_extension_handler.php
+++ b/libs/sysplugins/smarty_internal_extension_handler.php
@@ -88,20 +88,19 @@ public function _callExternalMethod(Smarty_Internal_Data $data, $name, $args)
                         $objType = $data->_objType;
                         $propertyType = false;
                         if (!isset($this->resolvedProperties[ $match[ 0 ] ][ $objType ])) {
-                            $property = isset($this->resolvedProperties[ 'property' ][ $basename ]) ?
-                                $this->resolvedProperties[ 'property' ][ $basename ] :
-                                $property = $this->resolvedProperties[ 'property' ][ $basename ] = strtolower(
-                                    join(
-                                        '_',
-                                        preg_split(
-                                            '/([A-Z][^A-Z]*)/',
-                                            $basename,
-                                            -1,
-                                            PREG_SPLIT_NO_EMPTY |
-                                            PREG_SPLIT_DELIM_CAPTURE
-                                        )
+                            $property = $this->resolvedProperties['property'][$basename] ??
+                                $this->resolvedProperties['property'][$basename] = smarty_strtolower_ascii(
+                                join(
+                                    '_',
+                                    preg_split(
+                                        '/([A-Z][^A-Z]*)/',
+                                        $basename,
+                                        -1,
+                                        PREG_SPLIT_NO_EMPTY |
+                                        PREG_SPLIT_DELIM_CAPTURE
                                     )
-                                );
+                                )
+                            );
                             if ($property !== false) {
                                 if (property_exists($data, $property)) {
                                     $propertyType = $this->resolvedProperties[ $match[ 0 ] ][ $objType ] = 1;
@@ -145,7 +144,7 @@ public function _callExternalMethod(Smarty_Internal_Data $data, $name, $args)
     public function upperCase($name)
     {
         $_name = explode('_', $name);
-        $_name = array_map('ucfirst', $_name);
+        $_name = array_map('smarty_ucfirst_ascii', $_name);
         return implode('_', $_name);
     }
 
diff --git a/libs/sysplugins/smarty_internal_method_loadplugin.php b/libs/sysplugins/smarty_internal_method_loadplugin.php
index 3bd659cb8..6ddcaec94 100644
--- a/libs/sysplugins/smarty_internal_method_loadplugin.php
+++ b/libs/sysplugins/smarty_internal_method_loadplugin.php
@@ -40,7 +40,7 @@ public function loadPlugin(Smarty $smarty, $plugin_name, $check)
             throw new SmartyException("plugin {$plugin_name} is not a valid name format");
         }
         if (!empty($match[ 2 ])) {
-            $file = SMARTY_SYSPLUGINS_DIR . strtolower($plugin_name) . '.php';
+            $file = SMARTY_SYSPLUGINS_DIR . smarty_strtolower_ascii($plugin_name) . '.php';
             if (isset($this->plugin_files[ $file ])) {
                 if ($this->plugin_files[ $file ] !== false) {
                     return $this->plugin_files[ $file ];
@@ -60,7 +60,7 @@ public function loadPlugin(Smarty $smarty, $plugin_name, $check)
         }
         // plugin filename is expected to be: [type].[name].php
         $_plugin_filename = "{$match[1]}.{$match[4]}.php";
-        $_lower_filename = strtolower($_plugin_filename);
+        $_lower_filename = smarty_strtolower_ascii($_plugin_filename);
         if (isset($this->plugin_files)) {
             if (isset($this->plugin_files[ 'plugins_dir' ][ $_lower_filename ])) {
                 if (!$smarty->use_include_path || $this->plugin_files[ 'plugins_dir' ][ $_lower_filename ] !== false) {
diff --git a/libs/sysplugins/smarty_internal_templatecompilerbase.php b/libs/sysplugins/smarty_internal_templatecompilerbase.php
index d6f86ac0a..545c1e4ff 100644
--- a/libs/sysplugins/smarty_internal_templatecompilerbase.php
+++ b/libs/sysplugins/smarty_internal_templatecompilerbase.php
@@ -605,7 +605,7 @@ public function compilePHPFunctionCall($name, $parameter)
             if (strcasecmp($name, 'isset') === 0 || strcasecmp($name, 'empty') === 0
                 || strcasecmp($name, 'array') === 0 || is_callable($name)
             ) {
-                $func_name = strtolower($name);
+                $func_name = smarty_strtolower_ascii($name);
 
                 if ($func_name === 'isset') {
                     if (count($parameter) === 0) {
@@ -765,7 +765,7 @@ public function getTagCompiler($tag)
         if (!isset(self::$_tag_objects[ $tag ])) {
             // lazy load internal compiler plugin
             $_tag = explode('_', $tag);
-            $_tag = array_map('ucfirst', $_tag);
+            $_tag = array_map('smarty_ucfirst_ascii', $_tag);
             $class_name = 'Smarty_Internal_Compile_' . implode('_', $_tag);
             if (class_exists($class_name)
                 && (!isset($this->smarty->security_policy) || $this->smarty->security_policy->isTrustedTag($tag, $this))
diff --git a/libs/sysplugins/smarty_resource.php b/libs/sysplugins/smarty_resource.php
index 7fe84536f..3c43a9f46 100644
--- a/libs/sysplugins/smarty_resource.php
+++ b/libs/sysplugins/smarty_resource.php
@@ -76,11 +76,11 @@ public static function load(Smarty $smarty, $type)
         }
         // try sysplugins dir
         if (isset(self::$sysplugins[ $type ])) {
-            $_resource_class = 'Smarty_Internal_Resource_' . ucfirst($type);
+            $_resource_class = 'Smarty_Internal_Resource_' . smarty_ucfirst_ascii($type);
             return $smarty->_cache[ 'resource_handlers' ][ $type ] = new $_resource_class();
         }
         // try plugins dir
-        $_resource_class = 'Smarty_Resource_' . ucfirst($type);
+        $_resource_class = 'Smarty_Resource_' . smarty_ucfirst_ascii($type);
         if ($smarty->loadPlugin($_resource_class)) {
             if (class_exists($_resource_class, false)) {
                 return $smarty->_cache[ 'resource_handlers' ][ $type ] = new $_resource_class();