Create WordPress widget

<?php
// Creazione Widget
class banner1 extends WP_Widget {
    function __construct() {
        parent::WP_Widget('Banner1', 'Banner #1', array('description' => 'Primo banner'));
    }

    function widget($args, $instance) {
        //estraggo variabili da args:
        extract($args);
        $str = "<div id='sponsor1' class='sponsor'><a href='";
        $str .= $instance['link'];
        $str .= "'>";
        if(!empty($instance['img'])) {
            $str .= "<img width='230' height='130' src='";
            $str .= get_site_url();
            $str .= "/wp-content/themes/atletico_luinese/images/";
            $str .= $instance['img'];
            $str .= "' />";
        }
        $str .= "</a></div>";
        echo $str;
    }

    function update($new_instance, $old_instance) {
        $instance = $old_instance;
        $instance['img'] = strip_tags($new_instance['img']);
        $instance['link'] = strip_tags($new_instance['link']);
        return $instance;
    }

    function form($instance) {
    if ( $instance ) {
        //codifico html
        $img = esc_attr($instance['img']);
        $link = esc_attr($instance['link']);
    }
    // Campi per modificare
    ?>
    <label for="<?php echo $this -> get_field_id('link');?>"><?php _e('Link:'); ?></label>
    <input id="<?php echo $this -> get_field_id('link');?>" class="widefat" type="text" name="<?php echo $this -> get_field_name('link');?>" value="<?php echo $link; ?>"
    <label for="<?php echo $this -> get_field_id('img');?>"><?php _e('Immagine:'); ?></label>
    <input id="<?php echo $this -> get_field_id('img');?>" class="widefat" type="text" name="<?php echo $this -> get_field_name('img');?>" value="<?php echo $img;?>" />
    <?php }
}

add_action('widgets_init', create_function('', 'register_widget("banner1");'));