Wordpress Taxonomy problem

php

<?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' );

the error message it shows is
Notice: Undefined variable: location in C:\XAMMP2\htdocs\rateIt\wp-content\plugins\job-listing.php on line 37
Notice: Array to string conversion in C:\XAMMP2\htdocs\rateIt\wp-includes\class-wp-taxonomy.php on line 360