diff --git a/misuzu.php b/misuzu.php
index ca415a8b..53e43b38 100644
--- a/misuzu.php
+++ b/misuzu.php
@@ -2,6 +2,7 @@
 namespace Misuzu;
 
 date_default_timezone_set('UTC');
+mb_internal_encoding('UTF-8');
 
 define('MSZ_DEBUG', file_exists(__DIR__ . '/vendor/phpunit/phpunit/composer.json'));
 
diff --git a/public/comments.php b/public/comments.php
index e907fce1..97b346bd 100644
--- a/public/comments.php
+++ b/public/comments.php
@@ -136,7 +136,7 @@ switch ($_GET['m'] ?? null) {
             break;
         }
 
-        if (strlen($commentText) > 5000) {
+        if (mb_strlen($commentText) > 5000) {
             echo render_info_or_json($isXHR, 'Your comment is too long.', 400);
             break;
         }
diff --git a/public/members.php b/public/members.php
index 0edbbed2..93ad7c26 100644
--- a/public/members.php
+++ b/public/members.php
@@ -7,8 +7,8 @@ $usersOffset = max((int)($_GET['o'] ?? 0), 0);
 $usersTake = 30;
 
 $roleId = (int)($_GET['r'] ?? MSZ_ROLE_MAIN);
-$orderBy = strtolower($_GET['ss'] ?? '');
-$orderDir = strtolower($_GET['sd'] ?? '');
+$orderBy = mb_strtolower($_GET['ss'] ?? '');
+$orderDir = mb_strtolower($_GET['sd'] ?? '');
 
 $orderDirs = [
     'asc' => 'Ascending',
diff --git a/public/settings.php b/public/settings.php
index b2f262dd..f3b0877c 100644
--- a/public/settings.php
+++ b/public/settings.php
@@ -242,7 +242,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                                             $settingsErrors[] = 'Unknown e-mail validation error.';
                                     }
                                 } else {
-                                    $updateAccountFields['email'] = strtolower($_POST['email']['new']);
+                                    $updateAccountFields['email'] = mb_strtolower($_POST['email']['new']);
                                     audit_log('PERSONAL_EMAIL_CHANGE', $app->getUserId(), [
                                         $updateAccountFields['email'],
                                     ]);
diff --git a/src/Application.php b/src/Application.php
index 5c6dd389..0dfbe45d 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -124,7 +124,7 @@ class Application extends ApplicationBase
      */
     public function getPath(string $path): string
     {
-        if (!starts_with($path, '/') && substr($path, 1, 2) !== ':\\') {
+        if (!starts_with($path, '/') && mb_substr($path, 1, 2) !== ':\\') {
             $path = __DIR__ . '/../' . $path;
         }
 
@@ -310,7 +310,7 @@ class Application extends ApplicationBase
         }
 
         if ($this->configInstance->contains('Mail')) {
-            $method = strtolower($this->configInstance->get('Mail', 'method'));
+            $method = mb_strtolower($this->configInstance->get('Mail', 'method'));
         }
 
         if (empty($method) || !array_key_exists($method, self::MAIL_TRANSPORT)) {
diff --git a/src/Config/ConfigManager.php b/src/Config/ConfigManager.php
index 792fc070..8859ca59 100644
--- a/src/Config/ConfigManager.php
+++ b/src/Config/ConfigManager.php
@@ -92,7 +92,7 @@ class ConfigManager
         switch (strtolower($type)) {
             case "bool":
             case "boolean":
-                $value = strlen($raw) > 0 && ($raw[0] === '1' || strtolower($raw) === "true");
+                $value = mb_strlen($raw) > 0 && ($raw[0] === '1' || mb_strtolower($raw) === "true");
                 break;
 
             case "int":
@@ -124,7 +124,7 @@ class ConfigManager
         $type = gettype($value);
         $store = null;
 
-        switch (strtolower($type)) {
+        switch (mb_strtolower($type)) {
             case 'boolean':
                 $store = $value ? '1' : '0';
                 break;
@@ -209,7 +209,7 @@ class ConfigManager
 
         foreach ($lines as $line) {
             $line = trim($line, "\r\n");
-            $length = strlen($line);
+            $length = mb_strlen($line);
 
             if ($length < 1
                 || starts_with($line, '#')
@@ -228,7 +228,7 @@ class ConfigManager
                 continue;
             }
 
-            if (strpos($line, '=') !== false) {
+            if (mb_strpos($line, '=') !== false) {
                 $split = explode('=', $line, 2);
 
                 if (count($split) < 2) {
@@ -238,7 +238,7 @@ class ConfigManager
                 $key = trim($split[0]);
                 $value = trim($split[1]);
 
-                if (strlen($key) > 0 && strlen($value) > 0) {
+                if (mb_strlen($key) > 0 && mb_strlen($value) > 0) {
                     $collection[$section][$key] = $value;
                 }
             }
diff --git a/src/DatabaseMigrationManager.php b/src/DatabaseMigrationManager.php
index b0d1cf10..b780be1a 100644
--- a/src/DatabaseMigrationManager.php
+++ b/src/DatabaseMigrationManager.php
@@ -146,7 +146,7 @@ final class DatabaseMigrationManager
             }
 
             for ($i = 4; $i < count($filenameSplit); $i++) {
-                $migrationName .= ucfirst(strtolower($filenameSplit[$i]));
+                $migrationName .= ucfirst(mb_strtolower($filenameSplit[$i]));
             }
 
             include_once $migration;
@@ -217,7 +217,7 @@ final class DatabaseMigrationManager
             $migrationName = '';
 
             for ($i = 4; $i < count($nameSplit); $i++) {
-                $migrationName .= ucfirst(strtolower($nameSplit[$i]));
+                $migrationName .= ucfirst(mb_strtolower($nameSplit[$i]));
             }
 
             include_once $migration['migration_path'];
diff --git a/src/ExceptionHandler.php b/src/ExceptionHandler.php
index 74d7fb76..f08c46b1 100644
--- a/src/ExceptionHandler.php
+++ b/src/ExceptionHandler.php
@@ -92,7 +92,7 @@ class ExceptionHandler
      */
     private static function report(Throwable $throwable): bool
     {
-        if (!strlen(self::$reportUrl)) {
+        if (!mb_strlen(self::$reportUrl)) {
             return false;
         }
 
@@ -119,7 +119,7 @@ class ExceptionHandler
             'Content-Type: application/json;charset=utf-8',
         ];
 
-        if (strlen(self::$reportSign)) {
+        if (mb_strlen(self::$reportSign)) {
             $headers[] = 'X-Misuzu-Signature: sha256=' . hash_hmac('sha256', $json, self::$reportSign);
         }
 
diff --git a/src/Forum/validate.php b/src/Forum/validate.php
index 850a397b..27b1e3f3 100644
--- a/src/Forum/validate.php
+++ b/src/Forum/validate.php
@@ -6,7 +6,7 @@ define('MSZ_POST_TEXT_LENGTH_MAX', 60000);
 
 function forum_validate_title(string $title): string
 {
-    $length = strlen($title);
+    $length = mb_strlen($title);
 
     if ($length < MSZ_TOPIC_TITLE_LENGTH_MIN) {
         return 'too-short';
@@ -21,7 +21,7 @@ function forum_validate_title(string $title): string
 
 function forum_validate_post(string $text): string
 {
-    $length = strlen($text);
+    $length = mb_strlen($text);
 
     if ($length < MSZ_POST_TEXT_LENGTH_MIN) {
         return 'too-short';
diff --git a/src/IO/Directory.php b/src/IO/Directory.php
index f45518bf..2027f8fd 100644
--- a/src/IO/Directory.php
+++ b/src/IO/Directory.php
@@ -102,7 +102,7 @@ class Directory
         foreach ($split_path as $path_part) {
             $existing_path .= $path_part . self::SEPARATOR;
 
-            if ($on_windows && substr($path_part, 1, 2) === ':\\') {
+            if ($on_windows && mb_substr($path_part, 1, 2) === ':\\') {
                 continue;
             }
 
diff --git a/src/Net/IPAddressRange.php b/src/Net/IPAddressRange.php
index 189e5797..7f9574c2 100644
--- a/src/Net/IPAddressRange.php
+++ b/src/Net/IPAddressRange.php
@@ -97,7 +97,7 @@ final class IPAddressRange
      */
     public static function fromMaskedString(string $maskedString): IPAddressRange
     {
-        if (strpos($maskedString, '/') === false) {
+        if (mb_strpos($maskedString, '/') === false) {
             throw new InvalidArgumentException('Invalid masked string.');
         }
 
@@ -118,7 +118,7 @@ final class IPAddressRange
      */
     public static function fromRangeString(string $rangeString): IPAddressRange
     {
-        if (strpos($rangeString, '-') === false) {
+        if (mb_strpos($rangeString, '-') === false) {
             throw new InvalidArgumentException('Invalid range string.');
         }
 
diff --git a/src/Users/validation.php b/src/Users/validation.php
index 21567d23..df42029e 100644
--- a/src/Users/validation.php
+++ b/src/Users/validation.php
@@ -16,7 +16,7 @@ define('MSZ_PASSWORD_MIN_ENTROPY', 32);
 
 function user_validate_username(string $username, bool $checkInUse = false): string
 {
-    $username_length = strlen($username);
+    $username_length = mb_strlen($username);
 
     if ($username !== trim($username)) {
         return 'trim';
diff --git a/src/changelog.php b/src/changelog.php
index b1833645..593ad808 100644
--- a/src/changelog.php
+++ b/src/changelog.php
@@ -11,7 +11,7 @@ function changelog_action_add(string $name, ?int $colour = null, ?string $class
         $colour = colour_none();
     }
 
-    $class = preg_replace('#[^a-z]#', '', strtolower($class ?? $name));
+    $class = preg_replace('#[^a-z]#', '', mb_strtolower($class ?? $name));
 
     $addAction = Database::prepare('
         INSERT INTO `msz_changelog_actions`
@@ -66,7 +66,7 @@ define('MSZ_CHANGELOG_GET_QUERY', '
 
 function changelog_get_changes(string $date, int $user, int $offset, int $take): array
 {
-    $hasDate = strlen($date) > 0;
+    $hasDate = mb_strlen($date) > 0;
     $hasUser = $user > 0;
 
     $query = sprintf(
@@ -101,7 +101,7 @@ define('CHANGELOG_COUNT_QUERY', '
 
 function changelog_count_changes(string $date, int $user): int
 {
-    $hasDate = strlen($date) > 0;
+    $hasDate = mb_strlen($date) > 0;
     $hasUser = $user > 0;
 
     $query = sprintf(
diff --git a/src/colour.php b/src/colour.php
index 224f2a51..536d1d12 100644
--- a/src/colour.php
+++ b/src/colour.php
@@ -115,10 +115,10 @@ function colour_from_rgb(int &$colour, int $red, int $green, int $blue): bool
 function colour_from_hex(int &$colour, string $hex): bool
 {
     if ($hex[0] === '#') {
-        $hex = substr($hex, 1);
+        $hex = mb_substr($hex, 1);
     }
 
-    $length = strlen($hex);
+    $length = mb_strlen($hex);
 
     if ($length === 3) {
         $hex = $hex[0] . $hex[0] . $hex[1] . $hex[1] . $hex[2] . $hex[2];
@@ -128,9 +128,9 @@ function colour_from_hex(int &$colour, string $hex): bool
 
     colour_from_rgb(
         $colour,
-        hexdec(substr($hex, 0, 2)),
-        hexdec(substr($hex, 2, 2)),
-        hexdec(substr($hex, 4, 2))
+        hexdec(mb_substr($hex, 0, 2)),
+        hexdec(mb_substr($hex, 2, 2)),
+        hexdec(mb_substr($hex, 4, 2))
     );
 
     return true;
diff --git a/src/zalgo.php b/src/zalgo.php
index d013291b..ba9e6145 100644
--- a/src/zalgo.php
+++ b/src/zalgo.php
@@ -74,7 +74,7 @@ function zalgo_run(
     int $mode = MSZ_ZALGO_MODE_MINI,
     int $direction = MSZ_ZALGO_DIR_MID | MSZ_ZALGO_DIR_DOWN
 ): string {
-    $text_length = strlen($text);
+    $text_length = mb_strlen($text);
 
     if (!$text_length || !$mode || !$direction) {
         return $text;