Define post_type search widget

I did not find a clue how to suggested Dang in the comments . I also thought about filtering the search results using hook posts_where , but I have no idea how to recognize if the search is being done by our widget or another search form.

I found a very simple way to filter the results that is putting an input field of type:

<input type="hidden" value="portfolio" name="post_type" />

Studying WP_Widget_Search I see that there is no way to modify it to include the filter, so the solution is to make a copy of the widget and adapt it. The modification I did was to remove get_search_form() and pull / modify the code for that same function .

PS: Adapt CPT to your need, here is being used portfolio .

<?php
/**
 * Plugin Name: (SOPT) Widget Search by CPT
 * Plugin URI: https://pt.stackoverflow.com/a/17233/201
 * Author: brasofilo
 */

add_action( 'widgets_init', 'b5f_sopt_search' );

function b5f_sopt_search() {
    register_widget( 'B5F_SOPT_Search_Widget' );
}

class B5F_SOPT_Search_Widget extends WP_Widget {
    function __construct() {
        $widget_ops = array('classname' => 'widget_search', 'description' => __( "A search form for your site.") );
        parent::__construct( 'search', _x( 'Search', 'Search widget' ), $widget_ops );
    }

    function widget( $args, $instance ) {
        extract($args);

        /** This filter is documented in wp-includes/default-widgets.php */
        $title = apply_filters( 'widget_title', empty( $instance['title'] ) ? '' : $instance['title'], $instance, $this->id_base );

        echo $before_widget;
        if ( $title )
            echo $before_title . $title . $after_title;

        $format = current_theme_supports( 'html5', 'search-form' ) ? 'html5' : 'xhtml';
        if ( 'html5' == $format ) {
            $form = '<form role="search" method="get" class="search-form" action="' . esc_url( home_url( '/' ) ) . '">
                <label>
                    <span class="screen-reader-text">' . _x( 'Search for:', 'label' ) . '</span>
                    <input type="search" class="search-field" placeholder="' . esc_attr_x( 'Search &hellip;', 'placeholder' ) . '" value="' . get_search_query() . '" name="s" title="' . esc_attr_x( 'Search for:', 'label' ) . '" />
                </label>
                <input type="hidden" value="portfolio" name="post_type" />
                <input type="submit" class="search-submit" value="'. esc_attr_x( 'Search', 'submit button' ) .'" />
            </form>';
        } else {
            $form = '<form role="search" method="get" id="searchform" class="searchform" action="' . esc_url( home_url( '/' ) ) . '">
                <div>
                    <label class="screen-reader-text" for="s">' . _x( 'Search for:', 'label' ) . '</label>
                    <input type="text" value="' . get_search_query() . '" name="s" id="s" />
                    <input type="hidden" value="portfolio" name="post_type" />
                    <input type="submit" id="searchsubmit" value="'. esc_attr_x( 'Search', 'submit button' ) .'" />
                </div>
            </form>';
        }
        echo $form;
        echo $after_widget;
    }

    function form( $instance ) {
        $instance = wp_parse_args( (array) $instance, array( 'title' => '') );
        $title = $instance['title'];
?>
        <p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?> <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>" /></label></p>
<?php
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        $new_instance = wp_parse_args((array) $new_instance, array( 'title' => ''));
        $instance['title'] = strip_tags($new_instance['title']);
        return $instance;
    }
}

Finally, modify your code to:

echo the_widget( 'B5F_SOPT_Search_Widget' );

Related: Can not limit search to only pages