plugin generated unexpected output during activation, but it is empty

Question

I am getting this error what is already answered in several threads here with same problem. Here the error message:

The plugin generated 3 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

Plugin that causes this error is written by me. To find out I deleted the complete code and only left the documentation block at the top.

Still get the error when activating my plugin even when it is empty! There is no cache running.

Find this very weird. How can an empty plugin generated the same error as when it is filled with code?

Someone of you any idea how to solve this? Would be very nice. I do not understand this.

Answer 1

You may have empty characters at the beginning before the <?php block.

You may also try to var_dump($result->get_error_data()) this error message — this would be the dirty way.

File: wp-admin/plugins.php
42:             $result = activate_plugin($plugin, self_admin_url('plugins.php?error=true&plugin=' . $plugin), is_network_admin() );
43:             if ( is_wp_error( $result ) ) {
44:                 if ( 'unexpected_output' == $result->get_error_code() ) {
45:                     $redirect = self_admin_url('plugins.php?error=true&charsout=' . strlen($result->get_error_data()) . '&plugin=' . $plugin . "&plugin_status=$status&paged=$page&s=$s");
46:                     wp_redirect(add_query_arg('_error_nonce', wp_create_nonce('plugin-activation-error_' . $plugin), $redirect));
47:                     exit;
48:                 } else {
49:                     wp_die($result);
50:                 }
51:             }