WP Admin Form Handler

Relevant part of callback for add_suubmenu_page()

    <form method="POST" action="<?php echo admin_url('admin-post.php?action=dc_convert'); ?>" novalidate="novalidate">
    <input type="hidden" id="_wpnonce" name="_wpnonce" value="<?php echo wp_create_nonce('dc_convert'); ?>" />

    The rest of input fields go here

    </form>

<?php
// Form submit handler
add_action('admin_post_dc_convert', 'dc_do_conversion');
function dc_do_conversion() {
?>
    <!DOCTYPE html>
    <html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="en-US">
    <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Conversion Result</title>
    </head>
    <body>
    <h1>Conversion Results</h1>
<?php
    if ( ! check_admin_referer('dc_convert')) wp_die('Invalid form data');
    if ( ! current_user_can('manage_options')) wp_die('You do not have proper user capability to do this.');
    if ( ! empty( $_POST )) {
        $source = stripslashes( $_POST['dc-source']);
        if (''== $source) {
            echo 'Content is empty!';
        } else {

        // do a bunch of processing

echo <<<EOT
$content
EOT;
// EOT; must be at col 0!

        } // if empty data
    } // if 'POST'
?>
</body>
</html>
<?php
    exit;
}