diff --git a/phpbb/phpbb/mention/source/friend.php b/phpbb/phpbb/mention/source/friend.php
index c0cbc261ce..8f78159b6c 100644
--- a/phpbb/phpbb/mention/source/friend.php
+++ b/phpbb/phpbb/mention/source/friend.php
@@ -32,6 +32,9 @@ class friend extends user
 		parent::__construct($db);
 	}
 
+	/**
+	 * {@inheritdoc}
+	 */
 	protected function query($keyword, $topic_id)
 	{
 		$query = $this->db->sql_build_query('SELECT', [
diff --git a/phpbb/phpbb/mention/source/source_interface.php b/phpbb/phpbb/mention/source/source_interface.php
new file mode 100644
index 0000000000..ace5cc9149
--- /dev/null
+++ b/phpbb/phpbb/mention/source/source_interface.php
@@ -0,0 +1,27 @@
+<?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\mention\source;
+
+interface source_interface
+{
+	/**
+	 * Searches database for names to mention
+	 * and returns and array of found items
+	 *
+	 * @param string $keyword  Search string
+	 * @param int    $topic_id Current topic ID
+	 * @return array Array of names
+	 */
+	public function get($keyword, $topic_id);
+}
diff --git a/phpbb/phpbb/mention/source/topic.php b/phpbb/phpbb/mention/source/topic.php
index 553abb07b6..1d72df711c 100644
--- a/phpbb/phpbb/mention/source/topic.php
+++ b/phpbb/phpbb/mention/source/topic.php
@@ -15,6 +15,9 @@ namespace phpbb\mention\source;
 
 class topic extends user
 {
+	/**
+	 * {@inheritdoc}
+	 */
 	protected function query($keyword, $topic_id)
 	{
 		$query = $this->db->sql_build_query('SELECT', [
diff --git a/phpbb/phpbb/mention/source/user.php b/phpbb/phpbb/mention/source/user.php
index 216fafa36d..be0d39f3af 100644
--- a/phpbb/phpbb/mention/source/user.php
+++ b/phpbb/phpbb/mention/source/user.php
@@ -13,7 +13,7 @@
 
 namespace phpbb\mention\source;
 
-abstract class user
+abstract class user implements source_interface
 {
 	/** @var \phpbb\db\driver\driver_interface */
 	protected $db;
@@ -26,8 +26,18 @@ abstract class user
 		$this->db = $db;
 	}
 
+	/**
+	 * Builds a query based on user input
+	 *
+	 * @param string $keyword  Search string
+	 * @param int    $topic_id Current topic ID
+	 * @return string Query ready for execution
+	 */
 	abstract protected function query($keyword, $topic_id);
 
+	/**
+	 * {@inheritdoc}
+	 */
 	public function get($keyword, $topic_id)
 	{
 		$keyword = utf8_clean_string($keyword);