Warning: Cannot modify header information

When I tried to add a new plugin it gives a warning like
Warning: Cannot modify header information headers already sent by (output started at –
**************/wp-content/plugins/imagepop.php:11) in ***************/wp-includes/pluggable.php on line 770

In my main php file in plugin I added only header plugin header information and one filter just as follows,

<php
/*
Plugin Name: imagepop
Description: A plugin that pops-up images when you mouse-over certain words.
Author: Dinishika
Version: 2.6
URL: http://dinishi.com
*/
?>
<?php add_filter(’the_content’, ‘imagpop_the_content’); function imagpop_the_content($content) { return $content; } ?>

I’m a newbie to the wordpress and like to play with it. Please help me.