Fix various compatibility issues with PHP 8

libs/ical/Property/ValueInterface.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of the eluceo/iCal package.
+ *
+ * (c) Markus Poerschke <markus@eluceo.de>
+ *
+ * This source file is subject to the MIT license that is bundled
+ * with this source code in the file LICENSE.
+ */
+
+namespace Eluceo\iCal\Property;
+
+interface ValueInterface
+{
+    /**
+     * Return the value of the Property as an escaped string.
+     *
+     * Escape values as per RFC 5545.
+     *
+     * @see https://tools.ietf.org/html/rfc5545#section-3.3.11
+     */
+    public function getEscapedValue(): string;
+}