In WordPress backend development, you may occasionally encounter the “headers already sent” error. This often occurs when a wp_redirect call is made after PHP has already sent HTTP headers to the browser. Since wp_redirect also functions by sending an HTTP header, it cannot execute if headers have already been transmitted.
Understanding the “Headers Already Sent” Issue
In a single HTTP request, only one set of HTTP headers can be sent to the browser. If headers have already been sent, any subsequent attempt by wp_redirect to send redirect headers will fail. In the HTTP protocol, sending headers is like pouring water out of a bottle—it cannot be undone. To help developers identify and fix this issue, PHP generates a warning when such an overlapping header request is made.
While this issue is technically a warning rather than a fatal error, it causes legitimate redirects to fail. Although you could simply disable error reporting to hide the warning, professional development requires resolving the underlying cause to ensure the program executes correctly and reliably.
How to Resolve the “Headers Already Sent” Problem
To successfully perform a page redirect, the redirect header must be sent before any other content or headers are transmitted to the browser. Let’s look at a code snippet that uses the admin_init hook. This hook executes after the WordPress administration interface has initialized. It is an ideal place to check for specific page parameters and perform redirects before any HTML output begins.
add_action('admin_init', function ()
{
if (isset($_GET[ 'page' ]) && $_GET[ 'page' ] === 'list_serial' && isset($_GET[ 'action' ]) && $_GET[ 'action' ] === 'delete') {
$ids = isset($_GET[ 'serial' ]) ? (array)$_GET[ 'serial' ] : [];
$sendback = remove_query_arg(['trashed', 'untrashed', 'deleted', 'locked', 'ids'], wp_get_referer());
wp_redirect(add_query_arg(['trashed' => count($ids), 'ids' => join('_', (array)$ids), 'locked' => 1], $sendback));
exit;
}
});This example is taken from a page using the WP_List_Table class. It handles data deletion from a list and then redirects back to the list page to show a success message. If the deletion and redirect logic were placed directly inside the WP_List_Table class methods—which execute after the page has already begun initializing and sending headers—it would trigger the “headers already sent” warning. By moving the logic to the admin_init hook, we ensure the redirect happens before any output starts.
While this example focuses on the WordPress backend, the same principle applies to the frontend. If you encounter similar issues on the site’s public-facing pages, simply use a frontend initialization hook (such as template_redirect) to handle your redirect logic before content is rendered.
