diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 12758b838..b842c53fc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,6 +33,7 @@ jobs:
           - "8.1"
           - "8.2"
           - "8.3"
+          - "8.4"
 
         compiler:
           - default
@@ -50,6 +51,9 @@ jobs:
           - os: ubuntu-latest
             php-version: "8.3"
             compiler: jit
+          - os: ubuntu-latest
+            php-version: "8.4"
+            compiler: jit
 
     steps:
     - name: Checkout
diff --git a/README.md b/README.md
index f73647e59..755d73ea0 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ Smarty is a template engine for PHP, facilitating the separation of presentation
 Read the [documentation](https://smarty-php.github.io/smarty/) to find out how to use it. 
 
 ## Requirements
-Smarty v5 can be run with PHP 7.2 to PHP 8.3.
+Smarty v5 can be run with PHP 7.2 to PHP 8.4.
 
 ## Installation
 Smarty versions 3.1.11 or later can be installed with [Composer](https://getcomposer.org/).
diff --git a/docker-compose.yml b/docker-compose.yml
index 7ecd8b436..de519b2ba 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -42,6 +42,11 @@ services:
       service: base
     build:
       dockerfile: ./utilities/testrunners/php83/Dockerfile
+  php84:
+    extends:
+      service: base
+    build:
+      dockerfile: ./utilities/testrunners/php84/Dockerfile
 volumes:
   smarty-code:
 
diff --git a/run-tests-for-all-php-versions.sh b/run-tests-for-all-php-versions.sh
index 494b127b5..efff5713d 100755
--- a/run-tests-for-all-php-versions.sh
+++ b/run-tests-for-all-php-versions.sh
@@ -14,3 +14,4 @@ $COMPOSE_CMD run --rm php80 ./run-tests.sh $@ && \
 $COMPOSE_CMD run --rm php81 ./run-tests.sh $@ && \
 $COMPOSE_CMD run --rm php82 ./run-tests.sh $@
 $COMPOSE_CMD run --rm php83 ./run-tests.sh $@
+$COMPOSE_CMD run --rm php84 ./run-tests.sh $@
diff --git a/src/Cacheresource/Base.php b/src/Cacheresource/Base.php
index 54a141921..41c4f6bc6 100644
--- a/src/Cacheresource/Base.php
+++ b/src/Cacheresource/Base.php
@@ -44,7 +44,7 @@ abstract public function populateTimestamp(Cached $cached);
 	 */
     abstract public function process(
 	    Template $_template,
-	    Cached   $cached = null,
+	    ?Cached  $cached = null,
 	             $update = false
     );
 
diff --git a/src/Cacheresource/Custom.php b/src/Cacheresource/Custom.php
index c049246c8..f9eb858e4 100644
--- a/src/Cacheresource/Custom.php
+++ b/src/Cacheresource/Custom.php
@@ -139,7 +139,7 @@ public function populateTimestamp(\Smarty\Template\Cached $cached)
 	 */
     public function process(
 	    Template               $_smarty_tpl,
-	    \Smarty\Template\Cached $cached = null,
+	    ?\Smarty\Template\Cached $cached = null,
 	                           $update = false
     ) {
         if (!$cached) {
diff --git a/src/Cacheresource/File.php b/src/Cacheresource/File.php
index 538f01034..4b4198ec2 100644
--- a/src/Cacheresource/File.php
+++ b/src/Cacheresource/File.php
@@ -99,7 +99,7 @@ public function populateTimestamp(Cached $cached)
 	 */
     public function process(
 	    Template $_smarty_tpl,
-	    Cached   $cached = null,
+	    ?Cached  $cached = null,
 	             $update = false
     ) {
         $_smarty_tpl->getCached()->setValid(false);
diff --git a/src/Cacheresource/KeyValueStore.php b/src/Cacheresource/KeyValueStore.php
index 733d27765..1953bb20f 100644
--- a/src/Cacheresource/KeyValueStore.php
+++ b/src/Cacheresource/KeyValueStore.php
@@ -103,7 +103,7 @@ public function populateTimestamp(Cached $cached)
 	 */
     public function process(
 	    Template $_smarty_tpl,
-	    Cached   $cached = null,
+	    ?Cached  $cached = null,
 	             $update = false
     ) {
         if (!$cached) {
diff --git a/src/Compiler/CodeFrame.php b/src/Compiler/CodeFrame.php
index 5e7821509..d0e1cc120 100644
--- a/src/Compiler/CodeFrame.php
+++ b/src/Compiler/CodeFrame.php
@@ -41,7 +41,7 @@ public function create(
         $content = '',
         $functions = '',
         $cache = false,
-        \Smarty\Compiler\Template $compiler = null
+        ?\Smarty\Compiler\Template $compiler = null
     ) {
         // build property code
         $properties[ 'version' ] = \Smarty\Smarty::SMARTY_VERSION;
diff --git a/src/Compiler/Template.php b/src/Compiler/Template.php
index 9b2c1a1f2..de802e1cc 100644
--- a/src/Compiler/Template.php
+++ b/src/Compiler/Template.php
@@ -374,7 +374,7 @@ public function compileTemplate(\Smarty\Template $template) {
 	 * @throws CompilerException
 	 * @throws Exception
 	 */
-	public function compileTemplateSource(\Smarty\Template $template, \Smarty\Compiler\Template $parent_compiler = null) {
+	public function compileTemplateSource(\Smarty\Template $template, ?\Smarty\Compiler\Template $parent_compiler = null) {
 		try {
 			// save template object in compiler class
 			$this->template = $template;
diff --git a/src/CompilerException.php b/src/CompilerException.php
index e3d67b46c..60af9db7c 100644
--- a/src/CompilerException.php
+++ b/src/CompilerException.php
@@ -16,14 +16,14 @@ class CompilerException extends Exception {
 	 * @param int $code The Exception code.
 	 * @param string|null $filename The filename where the exception is thrown.
 	 * @param int|null $line The line number where the exception is thrown.
-	 * @param Throwable|null $previous The previous exception used for the exception chaining.
+	 * @param \Throwable|null $previous The previous exception used for the exception chaining.
 	 */
 	public function __construct(
 		string    $message = "",
 		int       $code = 0,
 		?string   $filename = null,
 		?int      $line = null,
-		Throwable $previous = null
+		?\Throwable $previous = null
 	) {
 		parent::__construct($message, $code, $previous);
 
diff --git a/src/Resource/BasePlugin.php b/src/Resource/BasePlugin.php
index 56f7dfa05..6d2222237 100644
--- a/src/Resource/BasePlugin.php
+++ b/src/Resource/BasePlugin.php
@@ -112,7 +112,7 @@ abstract public function getContent(Source $source);
 	 * @param Source $source source object
 	 * @param Template|null $_template template object
 	 */
-    abstract public function populate(Source $source, \Smarty\Template $_template = null);
+    abstract public function populate(Source $source, ?\Smarty\Template $_template = null);
 
     /**
      * populate Source Object with timestamp and exists from Resource
diff --git a/src/Resource/CustomPlugin.php b/src/Resource/CustomPlugin.php
index b50ef7aaf..895e97108 100644
--- a/src/Resource/CustomPlugin.php
+++ b/src/Resource/CustomPlugin.php
@@ -50,7 +50,7 @@ protected function fetchTimestamp($name) {
 	 * @param Source $source source object
 	 * @param Template|null $_template template object
 	 */
-	public function populate(Source $source, Template $_template = null) {
+	public function populate(Source $source, ?Template $_template = null) {
 		$source->uid = sha1($source->type . ':' . $source->name);
 		$mtime = $this->fetchTimestamp($source->name);
 		if ($mtime !== null) {
diff --git a/src/Resource/ExtendsPlugin.php b/src/Resource/ExtendsPlugin.php
index 0f9c9de25..960e37971 100644
--- a/src/Resource/ExtendsPlugin.php
+++ b/src/Resource/ExtendsPlugin.php
@@ -23,7 +23,7 @@ class ExtendsPlugin extends BasePlugin
      *
      * @throws Exception
      */
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $uid = '';
         $sources = array();
diff --git a/src/Resource/FilePlugin.php b/src/Resource/FilePlugin.php
index c59595783..0033c8348 100644
--- a/src/Resource/FilePlugin.php
+++ b/src/Resource/FilePlugin.php
@@ -32,7 +32,7 @@ class FilePlugin extends BasePlugin {
 	 *
 	 * @throws Exception
 	 */
-	public function populate(Source $source, Template $_template = null) {
+	public function populate(Source $source, ?Template $_template = null) {
 
 		$source->uid = sha1(
 			$source->name . ($source->isConfig ? $source->getSmarty()->_joined_config_dir :
diff --git a/src/Resource/StreamPlugin.php b/src/Resource/StreamPlugin.php
index 91afffdcd..9b5b3f579 100644
--- a/src/Resource/StreamPlugin.php
+++ b/src/Resource/StreamPlugin.php
@@ -33,7 +33,7 @@ class StreamPlugin extends RecompiledPlugin {
 	 *
 	 * @return void
 	 */
-	public function populate(Source $source, Template $_template = null) {
+	public function populate(Source $source, ?Template $_template = null) {
 		$source->uid = false;
 		$source->content = $this->getContent($source);
 		$source->timestamp = $source->exists = !!$source->content;
diff --git a/src/Resource/StringPlugin.php b/src/Resource/StringPlugin.php
index 6b5bee4dd..fb3692c73 100644
--- a/src/Resource/StringPlugin.php
+++ b/src/Resource/StringPlugin.php
@@ -31,7 +31,7 @@ class StringPlugin extends BasePlugin {
 	 *
 	 * @return void
 	 */
-	public function populate(Source $source, Template $_template = null) {
+	public function populate(Source $source, ?Template $_template = null) {
 		$source->uid = sha1($source->name);
 		$source->timestamp = $source->exists = true;
 	}
diff --git a/src/Runtime/InheritanceRuntime.php b/src/Runtime/InheritanceRuntime.php
index ffd7aae66..74ea85440 100644
--- a/src/Runtime/InheritanceRuntime.php
+++ b/src/Runtime/InheritanceRuntime.php
@@ -162,7 +162,7 @@ public function instanceBlock(Template $tpl, $className, $name, $tplIndex = null
 	private function processBlock(
 		Template              $tpl,
 		\Smarty\Runtime\Block $block,
-		\Smarty\Runtime\Block $parent = null
+		?\Smarty\Runtime\Block $parent = null
 	) {
 		if ($block->hide && !isset($block->child)) {
 			return;
diff --git a/src/Template.php b/src/Template.php
index fcb0f58d2..242fb2388 100644
--- a/src/Template.php
+++ b/src/Template.php
@@ -115,7 +115,7 @@ class Template extends TemplateBase {
 	public function __construct(
 		$template_resource,
 		Smarty $smarty,
-		\Smarty\Data $_parent = null,
+		?\Smarty\Data $_parent = null,
 		$_cache_id = null,
 		$_compile_id = null,
 		$_caching = null,
@@ -248,7 +248,7 @@ public function renderSubTemplate(
 		$caching,
 		$cache_lifetime,
 		array $extra_vars = [],
-		int $scope = null,
+		?int $scope = null,
 		?string $currentDir = null
 	) {
 
@@ -462,7 +462,7 @@ public function getCompiler() {
 	 * @return string
 	 * @throws Exception
 	 */
-	public function createCodeFrame($content = '', $functions = '', $cache = false, \Smarty\Compiler\Template $compiler = null) {
+	public function createCodeFrame($content = '', $functions = '', $cache = false, ?\Smarty\Compiler\Template $compiler = null) {
 		return $this->getCodeFrameCompiler()->create($content, $functions, $cache, $compiler);
 	}
 
diff --git a/src/Template/Source.php b/src/Template/Source.php
index 382e710a7..2ef13a48c 100644
--- a/src/Template/Source.php
+++ b/src/Template/Source.php
@@ -134,9 +134,9 @@ public function __construct(Smarty $smarty, $type, $name) {
 	 * @throws Exception
 	 */
 	public static function load(
-		Template $_template = null,
-		Smarty   $smarty = null,
-		         $template_resource = null
+		?Template $_template = null,
+		?Smarty   $smarty = null,
+		          $template_resource = null
 	) {
 		if ($_template) {
 			$smarty = $_template->getSmarty();
diff --git a/src/TemplateBase.php b/src/TemplateBase.php
index 3674edf7c..f01d11076 100644
--- a/src/TemplateBase.php
+++ b/src/TemplateBase.php
@@ -179,7 +179,7 @@ public function setCacheId($cache_id) {
 	 * @api  Smarty::createData()
 	 *
 	 */
-	public function createData(Data $parent = null, $name = null) {
+	public function createData(?Data $parent = null, $name = null) {
 		/* @var Smarty $smarty */
 		$smarty = $this->getSmarty();
 		$dataObj = new Data($parent, $smarty, $name);
diff --git a/tests/PHPUnit_Smarty.php b/tests/PHPUnit_Smarty.php
index 18b4112fb..029c8f901 100644
--- a/tests/PHPUnit_Smarty.php
+++ b/tests/PHPUnit_Smarty.php
@@ -64,7 +64,7 @@ class PHPUnit_Smarty extends PHPUnit\Framework\TestCase
      */
     public static function setUpBeforeClass(): void
     {
-        error_reporting(E_ALL & ~E_STRICT & ~E_DEPRECATED & ~E_USER_DEPRECATED);
+        error_reporting(E_ALL & ~E_DEPRECATED & ~E_USER_DEPRECATED);
         self::$init = true;
         self::$pluginsdir =self::getSmartyPluginsDir();
     }
diff --git a/tests/UnitTests/CacheResourceTests/_shared/PHPunitplugins/resource.filetest.php b/tests/UnitTests/CacheResourceTests/_shared/PHPunitplugins/resource.filetest.php
index 0aa280c4e..40995a61e 100644
--- a/tests/UnitTests/CacheResourceTests/_shared/PHPunitplugins/resource.filetest.php
+++ b/tests/UnitTests/CacheResourceTests/_shared/PHPunitplugins/resource.filetest.php
@@ -12,7 +12,7 @@ class Smarty_Resource_FiletestPlugin extends FilePlugin
      * @param Source   $source    source object
      * @param Template $_template template object
      */
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         parent::populate($source, $_template);
         if ($source->exists) {
diff --git a/tests/UnitTests/ResourceTests/Custom/Ambiguous/PHPunitplugins/resource.ambiguous.php b/tests/UnitTests/ResourceTests/Custom/Ambiguous/PHPunitplugins/resource.ambiguous.php
index 85ffadd5c..0e5d173de 100644
--- a/tests/UnitTests/ResourceTests/Custom/Ambiguous/PHPunitplugins/resource.ambiguous.php
+++ b/tests/UnitTests/ResourceTests/Custom/Ambiguous/PHPunitplugins/resource.ambiguous.php
@@ -34,7 +34,7 @@ public function setSegment($segment)
      * @param Source   $source    source object
      * @param Template $_template template object
      */
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $segment = '';
         if ($this->segment) {
diff --git a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db.php b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db.php
index 593155043..9f983912c 100644
--- a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db.php
+++ b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db.php
@@ -16,7 +16,7 @@
 
 class Smarty_Resource_Db extends RecompiledPlugin {
 
-    public function populate(Source $source, Template $_template = null) {
+    public function populate(Source $source, ?Template $_template = null) {
         $source->uid = sha1($source->resource);
         $source->timestamp = 1000000000;
         $source->exists = true;
diff --git a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db2.php b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db2.php
index a4f654026..2eefc6c95 100644
--- a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db2.php
+++ b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db2.php
@@ -16,7 +16,7 @@
 
 class Smarty_Resource_Db2 extends RecompiledPlugin
 {
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $source->uid = sha1($source->resource);
         $source->timestamp = 0;
diff --git a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db3.php b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db3.php
index 455255e24..7bd904e25 100644
--- a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db3.php
+++ b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db3.php
@@ -15,7 +15,7 @@
 
 class Smarty_Resource_Db3 extends Smarty\Resource\BasePlugin
 {
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $source->uid = sha1($source->resource);
         $source->timestamp = 0;
diff --git a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db4.php b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db4.php
index 571df0d82..51162a0bf 100644
--- a/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db4.php
+++ b/tests/UnitTests/ResourceTests/ResourcePlugins/PHPunitplugins/resource.db4.php
@@ -16,7 +16,7 @@
 
 class Smarty_Resource_Db4 extends Smarty\Resource\BasePlugin
 {
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $source->uid = sha1($source->resource);
         $source->timestamp = 0;
diff --git a/tests/UnitTests/__shared/resources/resource.extendsall.php b/tests/UnitTests/__shared/resources/resource.extendsall.php
index 9ffddc05e..14d37908f 100644
--- a/tests/UnitTests/__shared/resources/resource.extendsall.php
+++ b/tests/UnitTests/__shared/resources/resource.extendsall.php
@@ -22,7 +22,7 @@ class My_Resource_Extendsall extends \Smarty\Resource\ExtendsPlugin
      *
      * @return void
      */
-    public function populate(Source $source, Template $_template = null)
+    public function populate(Source $source, ?Template $_template = null)
     {
         $uid = '';
         $sources = array();
diff --git a/utilities/testrunners/php84/Dockerfile b/utilities/testrunners/php84/Dockerfile
new file mode 100644
index 000000000..6050a3db0
--- /dev/null
+++ b/utilities/testrunners/php84/Dockerfile
@@ -0,0 +1,10 @@
+FROM php:8.4-rc-cli-bullseye
+
+## Basic utilities
+RUN apt-get update -yqq && apt-get install -y curl apt-utils git zip unzip
+
+## Composer
+COPY ./utilities/testrunners/shared/install-composer.sh /root/install-composer.sh
+WORKDIR /root
+RUN sh ./install-composer.sh
+RUN mv ./composer.phar /usr/local/bin/composer