How do I pass an argument to my custom walker?

Question

I've set up a custom walker, and it works as desired when I hard code my variable.

What I want is to be able to call my walker and pass it a variable that is used in the walker.

For example:

 'walker'          => new Plus_walker($refine => 'review'), 

The walker I've tweaked to output http://URL/review in order to find posts in the review category for this menu item. But I want to be able to use the same walker for 'previews', 'interviews' and more. Thus far my attempts at inputting variables have just broken the whole thing, 'Parse error: syntax error, unexpected T_DOUBLE_ARROW'.

I know I could just rewrite the code and make a few different walkers (Review_walker, Preview_walker etc), but this just seems wrong (and I figure this could be an important thing to know...)

My walker code is here:

class plus_walker extends Walker_Nav_Menu {

    var $refine;

        function __construct($refine) {

            $this->refine = $refine;
        }


      function start_el(&$output, $item, $depth, $args)
      {
           global $wp_query;
           $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

           $class_names = $value = '';

           $classes = empty( $item->classes ) ? array() : (array) $item->classes;

           $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
           $class_names = ' class="'. esc_attr( $class_names ) . '"';

           $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

           $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
           $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
           $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
           $attributes .= ! empty( $item->url )       ? ' href="'   . esc_attr( $item->url     )  . '' : ''; //This is where I want the argument to live.

    $modded_url = substr($attributes,0,-1) . '+' . $refine . '"';

            $item_output = $args->before;
            $item_output .= '<a'. $modded_url .'>';
            $item_output .= $args->link_before .$prepend.apply_filters( 'the_title', $item->title, $item->ID ).$append;
            $item_output .= $description.$args->link_after;
            $item_output .= '</a>';
            $item_output .= $args->after;

            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
            }
}

And the php call in my templates:

<?php wp_nav_menu( array(
                            'menu' => 'Handheld', 
                                'container'       => 'div', 
                                'container_class' => 'col_1', 
                                //'container_id'    => ,
                                'menu_class'      => 'menu', 
                                //  'menu_id'         => ,
                                'echo'            => true,
                                'fallback_cb'     => 'wp_page_menu',

                                'items_wrap'      => '<ul id="%1$s" class="%2$s">%3$s</ul>',
                                'depth'           => 0,
                                'walker'          => new Plus_walker(array($refine => 'review')), 


        )

        ); ?>

I've tried a bunch of different configurations: 'refine' => 'review, with and without the array, but still no joy, just errors (unexpected '=>' mostly).