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<?php 
// Custom contact form 7 retreat select
add_action( 'wpcf7_init', 'custom_retreat_select' );

function custom_retreat_select() {
    wpcf7_add_form_tag( 'retreat_select', 'custom_retreat_handler', array( 'name-attr' => true ) );
}

function custom_retreat_handler( $tag ) {
    $atts = array();
    $atts['name'] = $tag->name;
    $atts['class'] = $tag->get_class_option();
    $atts['id'] = $tag->get_id_option();
    $atts = wpcf7_format_atts( $atts );
    $html = '<select ' . $atts . '>';
    $args = array(
      'post_type' => 'retreats',
      'posts_per_page' => -1,
    );
    $retreats = get_posts( $args );
    foreach ( $retreats as $retreat ):
        $retreat_id = $retreat->ID;
        $slug = $retreat->post_name;
        $title = get_the_title($retreat_id);
        $html .= '<option value="' . $slug . '">' . $title . '</option>';
    endforeach;
    $html .= '</select>';
    return $html;
}