Need Help Determining Where Header Error is Happening

Question

I am currently writing a plugin that contains a report based on a mysql query. I would like to export this query information to CSV. Simple enough, I've done it many times outside of WordPress.

I'm currently receiving an error regarding headers already sent. I know what causes this error, but cannot find in my code where the issue is.

I have tried removing all other plugins, and even tried other plugins that also export CSV. The plugins that export CSV work just fine, and when my plugin is the only one activated, the error is still thrown so I am pretty sure it's an error on my part.

Also need to include that this is on a multisite network activated plugin.

The error points to the first header line:

    if (isset($_POST['txtExport'])) {
  header("Content-type: text/x-csv");
  header("Content-Disposition: attachment; filename=birts.csv");
  header("Pragma: no-cache");
  header("Expires: 0");

  foreach ($incidentReport as $vals) {
     echo "\"".$vals->student_id."\",\"".$vals->incident_desc."\",\"".$vals->incident_time."\",\"".$vals->incident_rating."\",\"".$vals->reported_by."\"\n<br/>";
  }
  exit;
    }

Below is the actual triggered error: Warning: Cannot modify header information - headers already sent by (output started at /home/plumwd/public_html/multisite/wp-admin/admin-header.php:28) in /home/plumwd/public_html/multisite/wp-content/plugins/bart/birts_dashboard.php on line 20

I have WP_DEBUG set to true and am showing no other errors.

Thanks!

Answer 1

I figured out what the problem was. In the end, I added &noheader=true to my form action. This allowed me to submit my own headers without calling the WP admin headers.

Answer 2

I've seen this now and again back in my days of developing in PHP. Usually when something comes up like this, it means that something in your script or some include of your script has already leaked a few characters out to be written and so the headers for the page has already been sent. I've seen that usually in this case that it could be something as simple as a space character...maybe even a carriage return (?)...that exists before the in one of these files. Sadly, if it's not in the main script itself, this usually means going through your dependent include files and hunting it down.