diff --git a/build/code_sniffer/phpbb/Sniffs/CodeLayout/UnionTypesCheckSniff.php b/build/code_sniffer/phpbb/Sniffs/CodeLayout/UnionTypesCheckSniff.php
new file mode 100644
index 0000000000..64b30a859e
--- /dev/null
+++ b/build/code_sniffer/phpbb/Sniffs/CodeLayout/UnionTypesCheckSniff.php
@@ -0,0 +1,116 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+namespace phpbb\Sniffs\CodeLayout;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+/**
+ * Checks that union type declarations follows the coding guidelines.
+ */
+class UnionTypesCheckSniff implements Sniff
+{
+	/**
+	* {@inheritdoc}
+	*/
+	public function register()
+	{
+		return [
+			T_FUNCTION,
+			T_CLASS,
+		];
+	}
+
+	/**
+	* {@inheritdoc}
+	*/
+	public function process(File $phpcsFile, $stackPtr)
+	{
+		$tokens = $phpcsFile->getTokens();
+		if ($tokens[$stackPtr]['type'] === 'T_FUNCTION')
+		{
+			$method_params = $phpcsFile->getMethodParameters($stackPtr);
+			$method_params_array = array_column($method_params, 'type_hint', 'token');
+			foreach ($method_params_array as $stack_pointer => $type_hint)
+			{
+				$this->check_union_type($phpcsFile, $stack_pointer, $type_hint);
+			}
+
+			$method_properties = $phpcsFile->getMethodProperties($stackPtr);
+			$this->check_union_type($phpcsFile, $stackPtr, $method_properties['return_type']);
+		}
+		else if ($tokens[$stackPtr]['type'] === 'T_CLASS')
+		{
+			$class_token = $tokens[$stackPtr];
+			$class_closer_pointer = $class_token['scope_closer'];
+			$first_method_pointer = $phpcsFile->findNext(T_FUNCTION, $stackPtr);
+			$class_members_declarations_end_pointer = $first_method_pointer ?: $class_closer_pointer;
+
+			$stack_pointer = $stackPtr;
+			while(($class_member_pointer = $phpcsFile->findNext(T_VARIABLE, $stack_pointer)) !== false && ($class_member_pointer < $class_members_declarations_end_pointer))
+			{
+				$properties = $phpcsFile->getMemberProperties($class_member_pointer);
+				$this->check_union_type($phpcsFile, $class_member_pointer, $properties['type']);
+				$stack_pointer = $class_member_pointer + 1;
+			}
+		}
+	}
+
+	public function check_union_type(File $phpcsFile, $stack_pointer, $type_hint)
+	{
+		if (empty($type_hint))
+		{
+			return;
+		}
+
+		if (!strpos($type_hint, '|') && $type_hint[0] == '?') // Check nullable shortcut syntax
+		{
+			$type = substr($type_hint, 1);
+			$error = 'Nullable shortcut syntax must not be used. Use union type instead: %1$s|null; found %2$s';
+			$data  = [$type, $type_hint];
+			$phpcsFile->addError($error, $stack_pointer, 'ShortNullableSyntax', $data);					
+		}
+		else if ((count($types_array = explode('|', $type_hint))) > 1) // Check union type layout
+		{
+			$types_array_null_less = $types_array;
+
+			// Check 'null' to be the last element
+			$null_position = array_search('null', $types_array);
+			if ($null_position !== false && $null_position != array_key_last($types_array))
+			{
+				$error = 'The "null" type hint must be the last of the union type elements; found %s';
+				$data  = [implode('|', $types_array)];
+				$phpcsFile->addError($error, $stack_pointer, 'NullAlwaysLast', $data);
+			}
+
+			// Check types excepting 'null' to follow alphabetical order
+			if ($null_position !== false)
+			{
+				array_splice($types_array_null_less, $null_position, 1);
+			}
+
+			if (count($types_array_null_less) > 1)
+			{
+				$types_array_null_less_sorted = $types_array_null_less;
+				sort($types_array_null_less_sorted);
+				if (!empty(array_diff_assoc($types_array_null_less, $types_array_null_less_sorted)))
+				{
+					$error = 'Union type elements must be sorted alphabetically excepting the "null" type hint must be the last if any; found %s';
+					$data  = [implode('|', $types_array)];
+					$phpcsFile->addError($error, $stack_pointer, 'AlphabeticalSort', $data);
+				}
+			}
+		}
+	}
+}
diff --git a/build/code_sniffer/ruleset-php-strict.xml b/build/code_sniffer/ruleset-php-strict.xml
index 9e2f0664d8..58fe270021 100644
--- a/build/code_sniffer/ruleset-php-strict.xml
+++ b/build/code_sniffer/ruleset-php-strict.xml
@@ -45,4 +45,8 @@
  <!-- There MUST NOT be unused use statements. -->
  <rule ref="./phpbb/Sniffs/Namespaces/UnusedUseSniff.php" />
 
+ <!-- The null type SHALL be the last union type element, other types SHALL follow alphabetical order.
+	No nullable type shortcut syntax should be used. -->
+ <rule ref="./phpbb/Sniffs/CodeLayout/UnionTypesCheckSniff.php" />
+
 </ruleset>
diff --git a/phpBB/phpbb/routing/loader_resolver.php b/phpBB/phpbb/routing/loader_resolver.php
index de3adcaa34..2e21d8bb3d 100644
--- a/phpBB/phpbb/routing/loader_resolver.php
+++ b/phpBB/phpbb/routing/loader_resolver.php
@@ -33,7 +33,7 @@ class loader_resolver implements LoaderResolverInterface
 	/**
 	 * {@inheritdoc}
 	 */
-	public function resolve($resource, $type = null): false|\Symfony\Component\Config\Loader\LoaderInterface
+	public function resolve($resource, $type = null): \Symfony\Component\Config\Loader\LoaderInterface|false
 	{
 		/** @var \Symfony\Component\Config\Loader\LoaderInterface $loader */
 		foreach ($this->loaders as $loader)