You will need a “form” and an “update” method for your widget class.
function form($instance) {
$instance = wp_parse_args(
(array) $instance,
array ('show')
);
$show = (!empty($instance['show'])) ? $instance['show'] : '';
echo '<input type="text" name="'.$this->get_field_name('show').'" value="'.esc_attr($show).'" />';
}
function update($new_instance, $old_instance) {
$instance = $old_instance;
$instance['show'] = ($new_instance['show']) ? $new_instance['show'] : 1;
return $instance;
}
The “form” method creates the back-end form. The “update” method updates the database. Most of the work is handled by the parent widget class.
Then use your ‘show’ variable– $instance['show'] to query for a post. I’ve included a default post to load.
function widget($args, $instance) { // widget sidebar output
extract($args, EXTR_SKIP);
echo $before_widget; // pre-widget code from theme
$pid = (!empty($instance['show'])) ? $instance['show'] : 123; // 123 is a default
$p = get_post($pid);
// echo your formatted post however you want
echo $after_widget; // post-widget code from theme
}