Fix various compatibility issues with PHP 8

libs/swiftmailer/classes/Swift/Mime/EncodingObserver.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of SwiftMailer.
+ * (c) 2004-2009 Chris Corbyn
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+/**
+ * Observes changes for a Mime entity's ContentEncoder.
+ *
+ * @author Chris Corbyn
+ */
+interface Swift_Mime_EncodingObserver
+{
+    /**
+     * Notify this observer that the observed entity's ContentEncoder has changed.
+     *
+     * @param Swift_Mime_ContentEncoder $encoder
+     */
+    public function encoderChanged(Swift_Mime_ContentEncoder $encoder);
+}