In PHP 5.0.0, is_a() became deprecated in favour of the instanceof operator. Calling is_a() would result in an E_STRICT warning.

In PHP 5.3.0, `is_a()` is no longer deprecated, and will therefore no longer throw `E_STRICT` warnings.

To avoid warnings in PHP < 5.3.0, convert all `is_a()` calls to `$var instanceof WP_Class` calls.

`instanceof` does not throw any error if the variable being tested is not an object, it simply returns `false`.

Props markoheijnen, wonderboymusic.
Fixes #25672.


git-svn-id: https://develop.svn.wordpress.org/trunk@31188 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/theme.php

@@ -149,8 +149,9 @@ function get_theme_update_available( $theme ) {
 	if ( !isset($themes_update) )
 		$themes_update = get_site_transient('update_themes');
 
-	if ( ! is_a( $theme, 'WP_Theme' ) )
+	if ( ! ( $theme instanceof WP_Theme ) ) {
 		return false;
+	}
 
 	$stylesheet = $theme->get_stylesheet();