diff --git a/src/pocketmine/MemoryManager.php b/src/pocketmine/MemoryManager.php
index 91f46d291..e61f9fb77 100644
--- a/src/pocketmine/MemoryManager.php
+++ b/src/pocketmine/MemoryManager.php
@@ -50,6 +50,7 @@ use function is_object;
 use function is_resource;
 use function is_string;
 use function json_encode;
+use function mb_strtoupper;
 use function min;
 use function mkdir;
 use function preg_match;
@@ -58,7 +59,6 @@ use function round;
 use function spl_object_hash;
 use function sprintf;
 use function strlen;
-use function strtoupper;
 use function substr;
 use const JSON_PRETTY_PRINT;
 use const JSON_UNESCAPED_SLASHES;
@@ -127,7 +127,7 @@ class MemoryManager{
 			if($m <= 0){
 				$defaultMemory = 0;
 			}else{
-				switch(strtoupper($matches[2])){
+				switch(mb_strtoupper($matches[2])){
 					case "K":
 						$defaultMemory = $m / 1024;
 						break;
diff --git a/src/pocketmine/entity/Effect.php b/src/pocketmine/entity/Effect.php
index 3471a0881..47447a7c9 100644
--- a/src/pocketmine/entity/Effect.php
+++ b/src/pocketmine/entity/Effect.php
@@ -31,7 +31,7 @@ use pocketmine\event\player\PlayerExhaustEvent;
 use pocketmine\utils\Color;
 use function constant;
 use function defined;
-use function strtoupper;
+use function mb_strtoupper;
 
 class Effect{
 	public const SPEED = 1;
@@ -102,7 +102,7 @@ class Effect{
 	}
 
 	public static function getEffectByName(string $name) : ?Effect{
-		$const = self::class . "::" . strtoupper($name);
+		$const = self::class . "::" . mb_strtoupper($name);
 		if(defined($const)){
 			return self::getEffect(constant($const));
 		}
diff --git a/src/pocketmine/event/EventPriority.php b/src/pocketmine/event/EventPriority.php
index a6d65003f..cdc63e8b3 100644
--- a/src/pocketmine/event/EventPriority.php
+++ b/src/pocketmine/event/EventPriority.php
@@ -25,7 +25,7 @@ namespace pocketmine\event;
 
 use function constant;
 use function defined;
-use function strtoupper;
+use function mb_strtoupper;
 
 /**
  * List of event priorities
@@ -79,7 +79,7 @@ abstract class EventPriority{
 	 * @throws \InvalidArgumentException
 	 */
 	public static function fromString(string $name) : int{
-		$name = strtoupper($name);
+		$name = mb_strtoupper($name);
 		$const = self::class . "::" . $name;
 		if($name !== "ALL" and defined($const)){
 			return constant($const);
diff --git a/src/pocketmine/item/ItemFactory.php b/src/pocketmine/item/ItemFactory.php
index 2d537231f..4993b454d 100644
--- a/src/pocketmine/item/ItemFactory.php
+++ b/src/pocketmine/item/ItemFactory.php
@@ -34,8 +34,8 @@ use function gettype;
 use function is_numeric;
 use function is_object;
 use function is_string;
+use function mb_strtoupper;
 use function str_replace;
-use function strtoupper;
 use function trim;
 
 /**
@@ -375,8 +375,8 @@ class ItemFactory{
 
 		if(is_numeric($b[0])){
 			$item = self::get((int) $b[0], $meta);
-		}elseif(defined(ItemIds::class . "::" . strtoupper($b[0]))){
-			$item = self::get(constant(ItemIds::class . "::" . strtoupper($b[0])), $meta);
+		}elseif(defined(ItemIds::class . "::" . mb_strtoupper($b[0]))){
+			$item = self::get(constant(ItemIds::class . "::" . mb_strtoupper($b[0])), $meta);
 		}else{
 			throw new \InvalidArgumentException("Unable to resolve \"" . $str . "\" to a valid item");
 		}
diff --git a/src/pocketmine/item/enchantment/Enchantment.php b/src/pocketmine/item/enchantment/Enchantment.php
index 8586cdc3e..4460e595f 100644
--- a/src/pocketmine/item/enchantment/Enchantment.php
+++ b/src/pocketmine/item/enchantment/Enchantment.php
@@ -26,7 +26,7 @@ namespace pocketmine\item\enchantment;
 use pocketmine\event\entity\EntityDamageEvent;
 use function constant;
 use function defined;
-use function strtoupper;
+use function mb_strtoupper;
 
 /**
  * Manages enchantment type data.
@@ -161,7 +161,7 @@ class Enchantment{
 	}
 
 	public static function getEnchantmentByName(string $name) : ?Enchantment{
-		$const = Enchantment::class . "::" . strtoupper($name);
+		$const = Enchantment::class . "::" . mb_strtoupper($name);
 		if(defined($const)){
 			return self::getEnchantment(constant($const));
 		}
diff --git a/src/pocketmine/plugin/PluginDescription.php b/src/pocketmine/plugin/PluginDescription.php
index 10db86877..e4332342e 100644
--- a/src/pocketmine/plugin/PluginDescription.php
+++ b/src/pocketmine/plugin/PluginDescription.php
@@ -29,12 +29,12 @@ use function array_values;
 use function constant;
 use function defined;
 use function is_array;
+use function mb_strtoupper;
 use function phpversion;
 use function preg_match;
 use function str_replace;
 use function stripos;
 use function strlen;
-use function strtoupper;
 use function substr;
 use function version_compare;
 use function yaml_parse;
@@ -147,7 +147,7 @@ class PluginDescription{
 		$this->prefix = (string) ($plugin["prefix"] ?? $this->prefix);
 
 		if(isset($plugin["load"])){
-			$order = strtoupper($plugin["load"]);
+			$order = mb_strtoupper($plugin["load"]);
 			if(!defined(PluginLoadOrder::class . "::" . $order)){
 				throw new PluginException("Invalid PluginDescription load");
 			}else{
diff --git a/src/pocketmine/plugin/PluginManager.php b/src/pocketmine/plugin/PluginManager.php
index 562657f36..e7861c8dc 100644
--- a/src/pocketmine/plugin/PluginManager.php
+++ b/src/pocketmine/plugin/PluginManager.php
@@ -59,12 +59,12 @@ use function is_dir;
 use function is_string;
 use function is_subclass_of;
 use function iterator_to_array;
+use function mb_strtoupper;
 use function mkdir;
 use function shuffle;
 use function stripos;
 use function strpos;
 use function strtolower;
-use function strtoupper;
 use const DIRECTORY_SEPARATOR;
 
 /**
@@ -379,7 +379,7 @@ class PluginManager{
 			if($version !== $serverString){
 				$pluginApi = array_pad(explode("-", $version, 2), 2, ""); //0 = version, 1 = suffix (optional)
 
-				if(strtoupper($pluginApi[1]) !== strtoupper($serverApi[1])){ //Different release phase (alpha vs. beta) or phase build (alpha.1 vs alpha.2)
+				if(mb_strtoupper($pluginApi[1]) !== mb_strtoupper($serverApi[1])){ //Different release phase (alpha vs. beta) or phase build (alpha.1 vs alpha.2)
 					continue;
 				}