diff --git a/wp-includes/wp-db.php b/wp-includes/wp-db.php
index 03e4e08910..68ac32c219 100644
--- a/wp-includes/wp-db.php
+++ b/wp-includes/wp-db.php
@@ -1314,6 +1314,10 @@ class wpdb {
 
 		$this->is_mysql = true;
 
+		/*
+		 * Deprecated in 3.9+ when using MySQLi. No equivalent
+		 * $new_link parameter exists for mysqli_* functions.
+		 */
 		$new_link = defined( 'MYSQL_NEW_LINK' ) ? MYSQL_NEW_LINK : true;
 		$client_flags = defined( 'MYSQL_CLIENT_FLAGS' ) ? MYSQL_CLIENT_FLAGS : 0;