表单发送不正确[重复]

I've been trying to make a form in Wordpress to post data to the database and check if the data matches and if so then it proceeds to the next form but I can't seem to get it to work properly, been testing having the form placed in the themes folder to allow straighter testing, the php looks like this:

<?php

if(isset($_POST['submit'])) {
  global $wpdb;

  $ordernumber = $_POST['ordernmbr'];
  $orderfirstname = $_POST['firstname'];
  $orderpostnumber = $_POST['postnmbr'];
  // Sanitizing
  $ordernumber = stripslashes_deep($ordernumber);
  $orderfirstname = stripslashes_deep($orderfirstname);
  $orderpostnumber = stripslashes_deep($orderpostnumber);


  $sql = "SELECT * FROM wp_postmeta WHERE 'post_id' = %d";
  $sql = $wpdb->prepare($sql, array($ordernumber));
  $res = $wpdb->get_results($sql);

  if ($res > 0) {
  wp_redirect(admin_url('http://localhost/wordpress/index.php/shop/'));
  die();

} else {
  $error = "Not like this";
  echo $error;
}
  print_r($res);
}
?>
<?php
 get_footer();
?>

    }

The problem is that when I try to post data it gives me an error saying

Warning: Cannot modify header information - headers already sent by (output started at /Applications/XAMPP/xamppfiles/htdocs/wordpress/wp-includes/class.wp-styles.php:225) in /Applications/XAMPP/xamppfiles/htdocs/wordpress/wp-includes/pluggable.php on line 1219

how do I go on about fixing this?

</div>

You can't redirect the browser after the content has already begun being served.

That piece of code needs to be included before the theme starts loading, so it needs to be in a plugin.

When setting up the plugin, be sure to give it a high priority, and choose the right filter/action to have it load early (before any content is rendered): WP Action Reference