Plugin getting Cannot modify header information errors

Question

I am trying to build my first plugin but am receiving the following error:

Warning: Cannot modify header information - headers already sent by (output started at /wp-admin/includes/template.php:1642) in /wp-includes/pluggable.php on line 876

But here's the thing: template.php isn't 1600 lines long and pluggable doesn't have any whitespace at that point.

My plugin code is as follows:

<?php  
/* 
Plugin Name: Test 
Plugin URI: http://www.Test.com
Description: Test
Author: Test
Version: 1.0 
Author URI: http://www.Test.com
*/  

function Test_admin() {  
    include('admin.php');  
}
function Test_admin_actions() { 
    $icon_url = "";
    add_menu_page( "Test", "Test", "switch_themes", "Test", "Test_admin", $icon_url, 58 );
}
add_action('admin_menu', 'Test_admin_actions');
?>

The admin.php file is currently empty.

The plugin adds a Test menu option in the admin page just before the Appearance option. When you click on it, you should theoretically get a white page but instead, you get the error.

So I'm pretty dumbfounded by this. I've disabled, deleted, and reinstalled my plugin to no avail.

Any ideas?

Answer 1

I'll guess your include is failing. If it's in the same directory as your main plugin file, try:

include( plugin_dir_path(__FILE__) . '/admin.php' );

Answer 2

It's most likely a space after the closing ?> tag. The WordPress Codex recommends that you drop the closing tag completely.