PHP7 is slated for release later this year. One of the changes is that PHP4 style constructors are deprecated. In order to prepare WordPress to support PHP7, these constructors have been deprecated in WordPress core. This is a quick guide to all you need to know about this change.

plugin.php

// Deprecating PHP4 style constructors in WordPress 4.3

class My_Widget extends WP_Widget {
    function __construct() {
        $widget_ops = array( 'classname' => 'css-class', 'description' => 'Description' );
        parent::__construct( 'css-class', 'Title', $widget_ops );
    }
    // Rest of your widget subclass goes here
}