PHP 7 - Class Method Compatibility Issue

Can anyone help me to determine what function/statement in this file causes the fatal error after upgrading to php 7.0? On php 5.6 everything works fine.

Error: "Declaration of theme_navigation::update() should be compatible with WP_Widget::update($new_instance, $old_instance)" on line 0

Code:

class theme_navigation extends WP_Widget {
public function __construct() {
    parent::__construct(
        'theme_navigation', // Base ID
        'Child Page Navigation', // Name
        array('description' => '') // Args
    );
}

function form($instance){
    $instance['tiutle'] = apply_filters( 'widget_title', $instance['title'] ); ?>
  <p>Title: <input name="cpn_title" id="cpn_title" type="text" <?php if(isset($instance['title']) && $instance['title'] != "") echo 'value="'.$instance['title'].'"'; ?> /></p>
  <p>Sort by Sort Order? <input name="cpn_sort" id="cpn_sort" type="checkbox" <?php if(isset($instance['sort']) && $instance['sort']) echo 'checked="checked"'; ?> value="1" /></p>
<?php }

function update(){
    if(!isset($_POST['cpn_sort']))
        $_POST['cpn_sort'] = 0;

    foreach($_POST as $field => $val){
        if(substr($field,0,4) != "cpn_")
            continue;

        $opts[substr($field,4)] = attribute_escape($_POST[$field]);
        unset($_POST[$field]);
    }

    return $opts;
}

function widget($args,$instance){
    global $post;
    $instance['title'] = apply_filters( 'widget_title', $instance['title'] );

    $page_arr = array('child_of' => $post->ID, 'parent' => $post->ID);

    if($instance['sort'])
        $page_arr['sort_column'] = 'menu_order';

    $pages = get_pages($page_arr);

    if($post->post_parent && !count($pages))
        $page_arr['child_of'] = $page_arr['parent'] = $post->post_parent;
        $pages = get_pages($page_arr);

    if(!count($pages))
        return;

    echo $args['before_widget'];

    if(isset($instance['title']) && $instance['title'] != "")
        echo $args['before_title'].$instance['title'].$args['after_title'];

    echo "<ul>";

    foreach($pages as $page){
        echo "<li><a href='".get_permalink($page->ID)."'>".get_the_title($page->ID)."</a></li>";
    }

    echo "</ul>".$args['after_widget'];
}
}
add_action( 'widgets_init', create_function( '', 'register_widget( "theme_navigation" );' ) );