Switch class properties from PHP typing to PHPDoc for compatibility with PHP < 7.4.0

classes/diskcache.php

@@ -1,6 +1,8 @@
 <?php
 class DiskCache {
-	private string $dir;
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var string */
+	private $dir;
 
 	/**
 	 * https://stackoverflow.com/a/53662733