I'm creating a WordPress Widget for show name of advertiser.

When I try to activate the plugin a error

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.

I did these things:

1- remove white space before or after the PHP opening or closing tags

2- change encoding to UTF8-BOM

class adsMain extends WP_Widget {

    public function __construct() {

            __( 'تبلیغات', 'text' ),
                'classname'   => 'adsMain',
                'description' => __( 'برای ایجاد تبلیغات جدید این کادر را به مکان دلخواه خود بکشید.', 'text' )
     * Front-end display of widget.
     * @see WP_Widget::widget()
     * @param array $args     Widget arguments.
     * @param array $instance Saved values from database.
    public function widget( $args, $instance ) {

        extract( $args );

        $advertiser = apply_filters( 'widget_title', $instance['advertiser'] );

        echo $advertiser;


      * Sanitize widget form values as they are saved.
      * @see WP_Widget::update()
      * @param array $new_instance Values just sent to be saved.
      * @param array $old_instance Previously saved values from database.
      * @return array Updated safe values to be saved.
    public function update( $new_instance, $old_instance ) {

        $instance = $old_instance;

        $instance['advertiser'] = strip_tags( $new_instance['advertiser'] );

        return $instance;


      * Back-end widget form.
      * @see WP_Widget::form()
      * @param array $instance Previously saved values from database.
    public function form( $instance ) {

        $advertiser = ( isset($instance['advertiser']) ? esc_attr ( $instance['advertiser'] ) : '' );

            <label for="<?php echo $this->get_field_id('advertiser'); ?>"><?php _e('نام شخص / شرکت تبلیغ دهنده:'); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id('advertiser'); ?>" name="<?php echo $this->get_field_name('advertiser'); ?>" type="text" value="<?php echo $advertiser; ?>" />


/* Register uploader */
add_action ('admin_enqueue_scripts', function () {
    wp_register_script('adsMain-script' , plugins_url( '/js/adsMain-script.js', __FILE__ ), array( 'jquery' ), '20160904', true );

/* Register the widget */
add_action('widgets_init', function() {
     register_widget( 'adsMain' );
  • 2
    try in UTF-8 without BOM
    – mmm
    Sep 4, 2016 at 8:41
  • @mmm please include your comment as an answer and try to add some explanation why you thought this would work.
    – cjbj
    Sep 4, 2016 at 14:53
  • If you are using a debugger, then put a breakpoint around in the activate_plugin method in wordpress/wp-admin/includes/plugin.php. The content of the $output variable might show you the data loaded into the Exception invariably being thrown, and then obfuscated by Wordpress. Feb 1, 2017 at 22:09

1 Answer 1


What is BOM?

The UTF-8 byte order mark or simply UTF-8 BOM is a series of bytes that helps the reader to understand the encoding of the current document.

These 3 bytes are:


Or simply the Unicode character : U+FEFF

In a plugin file encoded as BOM, these bytes will be output when the template is called, causing early output in header, that triggers a common error:

The plugin generated X characters of unexpected output during activation.

This can also happen to theme's files. Since using BOM for UTF-8 files is not recommended by Unicode Standards, you can save your templates as UTF-8 without BOM.

You will also notice that every UTF-8 file that is encoded under BOM has at least a size of 3 bytes, even if it's empty.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.