Export to CSV Base Code

This is useful if you are needing to create an admin exportable CSV file.  Use this as a base for your code to create an administrator only csv generator.

<?php


// Exit if accessed directly
if ( ! defined( 'ABSPATH' ) ) {
 exit;
}

add_action('admin_init', 'export_custom_query_to_csv');

function export_custom_query_to_csv() {
    if (!is_admin() || !current_user_can('manage_options')) return;

    if (!isset($_GET['export_custom_csv'], $_GET['_wpnonce'])) return;

    if (!wp_verify_nonce($_GET['_wpnonce'], 'export_custom_csv_action')) {
        wp_die('Security check failed.');
    }

    global $wpdb;

    $results = $wpdb->get_results("
        SELECT * 
        FROM table
        WHERE date >= CURDATE() - INTERVAL 30 DAY
    ", ARRAY_A);

    if (empty($results)) {
        wp_die('No data found to export.');
    }

    header('Content-Type: text/csv; charset=utf-8');
    header('Content-Disposition: attachment; filename=export-' . date('Y-m-d') . '.csv');

    $output = fopen('php://output', 'w');
    fputcsv($output, array_keys($results[0]));
    foreach ($results as $row) {
        fputcsv($output, $row);
    }
    fclose($output);
    exit;
}

add_action('admin_notices', 'show_csv_export_link');
function show_csv_export_link() {
    if (!current_user_can('manage_options')) return;

    $url = add_query_arg([
        'export_custom_csv' => 1,
        '_wpnonce' => wp_create_nonce('export_custom_csv_action'),
    ], admin_url());

    echo '<div class="notice notice-info"><p>';
    echo 'Download CSV: <a href="' . esc_url($url) . '" target="_blank">Click here to export</a>';
    echo '</p></div>';
}