Export CSV from Custom Table in wordpress

function exportleads_settings(){

    add_submenu_page(

        'options-general.php',

        __( 'Export Popup Leads', 'consumeracquision' ),

        __( 'Export Popup Leads', 'consumeracquision' ),

        'manage_options',

        'export-popup-feeds',

        'exportleads_callback'

    );

}

add_action('admin_menu', 'exportleads_settings'); 

function exportleads_callback() { 

    ?>

    <div class="wrap">

    <h2><?php _e( 'Export Popup Leads', 'textdomain' ); ?></h2>

    <a href="<?php  echo admin_url(); ?>admin-post.php?action=our_action_hook">Export Popup Leads</a>

    </div>

    <?php

}

add_action( 'admin_post_our_action_hook', 'am_our_action_hook_function' );

function am_our_action_hook_function() {

    if ( ! current_user_can( 'manage_options' ) )

        return;

    $csv_filename = 'Leads_'.date('D M d Y H:i:s A e',time()).'.csv' ;

    header('Content-Type: application/csv');

    header('Content-Disposition: attachment; filename='.$csv_filename);

    header('Pragma: no-cache');

    header('Expires: 0');

    $file = fopen('php://output', 'w');

    // send the column headers

    fputcsv($file, array('S.No','Email Address'),',');

        global $wpdb;

        $sql = 'SELECT * FROM '.$wpdb->prefix.'leads';

        $users = $wpdb->get_results( $sql, 'ARRAY_A' );

        $i=1;

        foreach ( $users as $user ) {

            fputcsv($file, array($i, $user['email']  ) );

            $i++;

        }

        exit();

}