<?php

add_filter('gform_filter_ui', 'referring_member_ui', 10, 5);

function referring_member_ui($filter_ui, $form_id, $page_title, $gf_page, $gf_view ) {

    if($form_id != '6' || $page_title != 'Sales') {
        return $filter_ui;
    }

    $markup = '<div style="width:100%;">
            <select id="gaddon-sales-referring-member" name="v[]">
                <option value="">-- Select --</option>';

    $members_so_far = get_referring_members();
    foreach ($members_so_far as $key => $member) {
        $markup .= '<option value="'.$member.'">' . $member . '</option>';
    }

    $markup .= '</select></div>';
    $markup .= '<input type="hidden" name="f[]" value="17">';
    $markup .= '<input type="hidden" name="o[]" value="is">';

    $filter_ui['referring_member'] = array(
        'label'   => '<br>Referring Member',
        'tooltip' => 'gresults_referring_member',
        'markup'  => $markup
    );

    return $filter_ui;
}

function get_referring_members() {

    $form_id = 6;
    $search_criteria['field_filters'][] = array( 'key' => '23', 'value' => 'Yes' );
    $entries = GFAPI::get_entries($form_id, $search_criteria);

    $members_so_far = wp_list_pluck($entries, '17');
    $members_so_far = array_unique($members_so_far);
    sort($members_so_far);

    return $members_so_far;

}