diff --git a/src/ExpressionParser.php b/src/ExpressionParser.php
index 4304af8bf..6784ebfd1 100644
--- a/src/ExpressionParser.php
+++ b/src/ExpressionParser.php
@@ -638,7 +638,7 @@ class ExpressionParser
                 $stream->expect(/* Token::NAME_TYPE */ 5, null, 'Only variables can be assigned to');
             }
             $value = $token->getValue();
-            if (\in_array(strtolower($value), ['true', 'false', 'none', 'null'])) {
+            if (\in_array(strtr($value, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), ['true', 'false', 'none', 'null'])) {
                 throw new SyntaxError(sprintf('You cannot assign a value to "%s".', $value), $token->getLine(), $stream->getSourceContext());
             }
             $targets[] = new AssignNameExpression($value, $token->getLine());
diff --git a/src/Extension/CoreExtension.php b/src/Extension/CoreExtension.php
index 3f62547d2..bbb55596b 100644
--- a/src/Extension/CoreExtension.php
+++ b/src/Extension/CoreExtension.php
@@ -1457,7 +1457,7 @@ function twig_get_attribute(Environment $env, Source $source, $object, $item, ar
     if (!isset($cache[$class])) {
         $methods = get_class_methods($object);
         sort($methods);
-        $lcMethods = array_map('strtolower', $methods);
+        $lcMethods = array_map(function ($value) { return strtr($value, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'); }, $methods);
         $classCache = [];
         foreach ($methods as $i => $method) {
             $classCache[$method] = $method;
@@ -1496,7 +1496,7 @@ function twig_get_attribute(Environment $env, Source $source, $object, $item, ar
     $call = false;
     if (isset($cache[$class][$item])) {
         $method = $cache[$class][$item];
-    } elseif (isset($cache[$class][$lcItem = strtolower($item)])) {
+    } elseif (isset($cache[$class][$lcItem = strtr($item, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz')])) {
         $method = $cache[$class][$lcItem];
     } elseif (isset($cache[$class]['__call'])) {
         $method = $item;
diff --git a/src/Sandbox/SecurityPolicy.php b/src/Sandbox/SecurityPolicy.php
index ca7c30738..2fc0d0131 100644
--- a/src/Sandbox/SecurityPolicy.php
+++ b/src/Sandbox/SecurityPolicy.php
@@ -50,7 +50,7 @@ final class SecurityPolicy implements SecurityPolicyInterface
     {
         $this->allowedMethods = [];
         foreach ($methods as $class => $m) {
-            $this->allowedMethods[$class] = array_map('strtolower', \is_array($m) ? $m : [$m]);
+            $this->allowedMethods[$class] = array_map(function ($value) { return strtr($value, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'); }, \is_array($m) ? $m : [$m]);
         }
     }
 
@@ -92,7 +92,7 @@ final class SecurityPolicy implements SecurityPolicyInterface
         }
 
         $allowed = false;
-        $method = strtolower($method);
+        $method = strtr($method, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz');
         foreach ($this->allowedMethods as $class => $methods) {
             if ($obj instanceof $class) {
                 $allowed = \in_array($method, $methods);