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-includes/theme.php

@@ -2027,5 +2027,5 @@ function wp_customize_support_script() {
 function is_customize_preview() {
 	global $wp_customize;
 
-	return is_a( $wp_customize, 'WP_Customize_Manager' ) && $wp_customize->is_preview();
+	return ( $wp_customize instanceof WP_Customize_Manager ) && $wp_customize->is_preview();
 }