Wordpress 분류 오류

<?php
/*
Plugin Name: job list
Plugin URI: http://example.com/
Description: A complete and practical example of use of the Settings API. This plugin creates a new plugin administration page.
Author: WROX
Author URI: http://wrox.com
*/
function dwwp_register_taxonomy() {
    $singular = 'Location';
    $plural = 'Locations';
    $slug = str_replace( ' ', '_', strtolower( $singular ) );
    $labels = array(
        'name'                       => $plural,
        'singular_name'              => $singular,
        'search_items'               => 'Search ' . $plural,
        'popular_items'              => 'Popular ' . $plural,
        'all_items'                  => 'All ' . $plural,
        'parent_item'                => null,
        'parent_item_colon'          => null,
        'edit_item'                  => 'Edit ' . $singular,
        'update_item'                => 'Update ' . $singular,
        'add_new_item'               => 'Add New ' . $singular,
        'new_item_name'              => 'New ' . $singular . ' Name',
        'separate_items_with_commas' => 'Separate ' . $plural . ' with commas',
        'add_or_remove_items'        => 'Add or remove ' . $plural,
        'choose_from_most_used'      => 'Choose from the most used ' . $plural,
        'not_found'                  => 'No ' . $plural . ' found.',
        'menu_name'                  => $plural,
    );
    $args = array(
            'hierarchical'          => true,
            'labels'                => $labels,
            'show_ui'               => true,
            'show_admin_column'     => true,
            'query_var'             => true,
            'rewrite'               => array( 'slug' => $$slug ),
        );
    register_taxonomy( 'location', $args );
}
add_action( 'init', 'dwwp_register_taxonomy' );

나는 워드 프레스 플러그인을 만들고 분류를 잘 모른다. 그것은 다음과 같은 오류를 준다.주의 : 정의되지 않은 변수 : C : \ XAMMP2 \ htdocs \ rateIt \ wp-content \ plugins \ job-listing.php on line 37 ( 이것은 37 행 = register_taxonomy ( 'location', $ args);)주의 : 360 행 C : \ XAMMP2 \ htdocs \ rateIt \ wp-includes \ class-wp-taxonomy.php의 배열에서 문자열로 변환