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..c74ec56128
--- /dev/null
+++ b/build/code_sniffer/phpbb/Sniffs/CodeLayout/UnionTypesCheckSniff.php
@@ -0,0 +1,96 @@
+<?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_TYPE_UNION,
+			T_NULLABLE,
+		];
+	}
+
+	/**
+	 * {@inheritdoc}
+	 */
+	public function process(File $phpcsFile, $stackPtr)
+	{
+		$tokens = $phpcsFile->getTokens();
+
+		// Check if nullable type shortcut syntax ('?typehint') is used
+		if ($tokens[$stackPtr]['type'] === 'T_NULLABLE')
+		{
+			$type = $tokens[$stackPtr + 1]['content'];
+			$error = 'Nullable shortcut syntax must not be used. Use union type instead: %2$s|null; found %1$s%2$s';
+			$data  = [trim($tokens[$stackPtr]['content']), $type];
+			$phpcsFile->addError($error, $stackPtr, 'ShortNullableSyntax', $data);
+		}
+
+		// Get the entry before the 1st '|' and all entries after it untill the end of union type declaration
+        if ($tokens[$stackPtr]['type'] === 'T_TYPE_UNION' && $tokens[$stackPtr - 2]['type'] !== 'T_TYPE_UNION')
+		{
+			// Get all the types within the union type declaration
+			$types_array = [];
+			for ($i = $stackPtr - 2; $i++;)
+			{
+				if (in_array($tokens[$i]['type'], ['T_TYPE_UNION', 'T_STRING', 'T_NULL']))
+				{
+					if ($tokens[$i]['type'] != 'T_TYPE_UNION')
+					{
+						$types_array[] = $tokens[$i]['content'];
+					}
+				}
+				else
+				{
+					break;
+				}
+			}
+
+			$types_array_sorted = $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, $stackPtr, 'NullAlwaysLast', $data);
+			}
+
+			// Check types excepting 'null' to follow alphabetical order
+			if ($null_position !== false)
+			{
+				array_splice($types_array_null_less, $null_position, 1);
+			}
+			sort($types_array_sorted);
+			if (!empty(array_diff_assoc($types_array_null_less, $types_array_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, $stackPtr, '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>