In `Walker_CategoryDropdown::start_el()`, cast values to strings before deciding whether to append 'selected' attribute.

As of [32484], `wp_dropdown_categories()` uses the `$value_field` value to
decide whether a given `<option>` should be 'selected'. However, `$value_field`
can refer to a value that is a string, such as a category's slug. This causes
problems when doing a loose comparison (`==`) with the value of the 'selected'
parameter, which defaults to `0`, because when doing a loose comparison
between an integer and a string, PHP will cast the string to an integer. This
creates false matches, resulting in `<option>` elements getting a 'selected'
attribute incorrectly.

We address the issue by casting the comparison values to strings, and then
using the strict comparison operator `===`.

Merges [33681] to the 4.3 branch.

Fixes #33452 for 4.3.1.

Built from https://develop.svn.wordpress.org/branches/4.3@33949


git-svn-id: http://core.svn.wordpress.org/branches/4.3@33918 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/category-template.php

@@ -1207,7 +1207,8 @@ class Walker_CategoryDropdown extends Walker {
 
 		$output .= "\t<option class=\"level-$depth\" value=\"" . esc_attr( $category->{$value_field} ) . "\"";
 
-		if ( $category->{$value_field} == $args['selected'] )
+		// Type-juggling causes false matches, so we force everything to a string.
+		if ( (string) $category->{$value_field} === (string) $args['selected'] )
 			$output .= ' selected="selected"';
 		$output .= '>';
 		$output .= $pad.$cat_name;