General: In the is_countable() polyfill, if the provided object implements SimpleXMLElement or ResourceBundle, consider it countable.

Props ayeshrajans, jrf, desrosj.
Fixes #43583.

git-svn-id: https://develop.svn.wordpress.org/trunk@43220 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/compat.php

@@ -520,7 +520,11 @@ if ( ! function_exists( 'is_countable' ) ) {
 	 * @return bool True if `$var` is countable, false otherwise.
 	 */
 	function is_countable( $var ) {
-		return ( is_array( $var ) || $var instanceof Countable );
+		return ( is_array( $var )
+			|| $var instanceof Countable
+			|| $var instanceof SimpleXMLElement
+			|| $var instanceof ResourceBundle
+		);
 	}
 }