diff --git a/php/Dockerfile-8.6 b/php/Dockerfile-8.6
index cd8f53c..d9ad91b 100644
--- a/php/Dockerfile-8.6
+++ b/php/Dockerfile-8.6
@@ -26,6 +26,7 @@ ARG EXTENSION_DEV_DEPS=" \
         libicu-dev \
         libjpeg62-turbo-dev \
 #       libmemcached-dev \
+        libonig-dev \
         libpng-dev \
         libsodium-dev \
         libssl-dev \
@@ -50,6 +51,7 @@ ARG EXTENSION_RUNTIME_DEPS="  \
         libjpeg62-turbo \
         liblzma5 \
 #       libmemcached11t64 \
+        libonig5 \
         libpng16-16t64 \
         librav1e0.7 \
         libsharpyuv0 \
@@ -90,6 +92,7 @@ RUN set -eux; \
         gettext \
         gmp \
         intl \
+        mbstring \
         mysqli \
         pcntl \
         pdo_mysql \
diff --git a/php/Dockerfile-8.6-cli b/php/Dockerfile-8.6-cli
index d6d5d59..5179739 100644
--- a/php/Dockerfile-8.6-cli
+++ b/php/Dockerfile-8.6-cli
@@ -26,6 +26,7 @@ ARG EXTENSION_DEV_DEPS=" \
         libicu-dev \
         libjpeg62-turbo-dev \
 #       libmemcached-dev \
+        libonig-dev \
         libpng-dev \
         libsodium-dev \
         libssl-dev \
@@ -50,6 +51,7 @@ ARG EXTENSION_RUNTIME_DEPS="  \
         libjpeg62-turbo \
         liblzma5 \
 #       libmemcached11t64 \
+        libonig5 \
         libpng16-16t64 \
         librav1e0.7 \
         libsharpyuv0 \
@@ -90,6 +92,7 @@ RUN set -eux; \
         gettext \
         gmp \
         intl \
+        mbstring \
         mysqli \
         pcntl \
         pdo_mysql \
diff --git a/tools/tests/Extensions/MbstringTest.php b/tools/tests/Extensions/MbstringTest.php
new file mode 100644
index 0000000..ebdc596
--- /dev/null
+++ b/tools/tests/Extensions/MbstringTest.php
@@ -0,0 +1,24 @@
+<?php
+/** @noinspection PhpComposerExtensionStubsInspection */
+
+declare(strict_types=1);
+
+use Tester\Assert;
+use Tester\Environment;
+
+require __DIR__ . '/../bootstrap.php';
+
+if (!extension_loaded('mbstring')) {
+    Environment::skip('Extension mbstring is not loaded');
+}
+
+$text = 'Žluťoučký kůň 🐎';
+Assert::same(15, mb_strlen($text, 'UTF-8'));
+Assert::same('kůň', mb_substr($text, 10, 3, 'UTF-8'));
+Assert::same(10, mb_strpos($text, 'kůň', 0, 'UTF-8'));
+Assert::same('ŽLUŤOUČKÝ KŮŇ 🐎', mb_strtoupper($text, 'UTF-8'));
+Assert::true(mb_check_encoding($text, 'UTF-8'));
+Assert::false(mb_check_encoding("\xC3\x28", 'UTF-8'));
+
+Assert::same("\x00\x41\x01\x7D", mb_convert_encoding('AŽ', 'UTF-16BE', 'UTF-8'));
+Assert::same('AŽ', mb_convert_encoding("\x00\x41\x01\x7D", 'UTF-8', 'UTF-16BE'));