Getting warning in a plugin for 'Cannot modify header information'

Question

I am making a plugin for Question2Answer and in that I am getting warnings as such

Warning: Cannot modify header information - headers already sent by (output started at /home/name/public_html/test/qa-plugin/welcome-widget/qa-welcome-widget.php:2) in /home/name/public_html/test/qa-include/qa-page.php on line 767

I searched about it and found that white spaces or rendering HTML before content type or encoding can be an issue. I tried to go through all of these options but couldn't figure out the problem. If possible then, please point me to the right direction.

Here's the code: '

<link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.6.3/css/font-awesome.min.css">
<link rel="stylesheet" type="text/css" href="<?php echo qa_path_to_root() ?>/qa-plugin/Welcome-Widget/welcome-widget.css">
<script type="text/javascript" href="<?php echo qa_path_to_root() ?>/qa-plugin/Welcome-Widget/js/qa-welcome-widget.js"></script>
<?php
class qa_welcome_widget {
    function allow_template($template)
    {
        return true;
    }
    function allow_region($region)
    {
        return true;
    }
    function output_widget($region, $place, $themeobject, $template, $request, $qa_content)
    {
        echo    '<div class="ww-wrapper">';
        echo            '<p id="ww-heading">'.qa_lang_html('plugin_ww/heading_option').'</p>';
        echo                '<table class="ww-box">';
        echo                    '<tr>';
        echo                        '<td class="ww-question"><i class="fa fa-question-circle" aria-hidden="true"></i>';
        echo                        '<p>'.qa_lang_html('plugin_ww/first_option').'</p>';
        echo                        '<p><a href="/ask" class="ww-button">Ask a Question</a></p></td>';
        echo                        '<td class="ww-reply"><i class="fa fa-comment" aria-hidden="true"></i>';
        echo                        '<p>'.qa_lang_html('plugin_ww/second_option').'</p>';
        echo                        '<p><a href="/questions" class="ww-button">View Questions</a></p></td>';
        echo                        '<td class="ww-vote"><i class="fa fa-thumbs-up" aria-hidden="true"></i>';
        echo                        '<p>'.qa_lang_html('plugin_ww/third_option').'</p>';
        echo                        '<p><a href="/questions" class="ww-button">View Questions</a></p></td>';
        echo                    '</tr>';
        echo                '</table>';
        echo    '</div>';
        echo           '<ul class="ww-topnav" id="ww-myTopnav">
                            <li><p>Menu -></p></li>
                            <li><a href="/questions">Recent</a></li>
                            <li><a href="/hot">Hot</a></li>
                            <li><a href="/questions?sort=votes">Most Votes</a></li>
                            <li><a href="questions?sort=answers">Most Answers</a></li>
                            <li><a href="questions?sort=views">Most Views</a></li>
                            <li class="icon">
                                <a href="javascript:void(0);" onclick="myFunction()">&#9776;</a>
                            </li>
                        </ul>';
    }
}

For the complete code you can see the plugin on Github.