Dynamic Sidebars & Echo

You don’t need to echo dynamic_sidebar(). End your echo, call dynamic_sidebar(), and then start a new echo. To make it easier, just close/reopen your PHP tags:

<?php
if( fake if code ) {
    if( another fake if code ) {
        ?>
        <div class="sidebar sidebar-first">
            <ul class="widget-area first">
                <?php dynamic_sidebar( 'pop-news-sidebar' ); ?>
            </ul>
        </div>
        <?php
    } else if( fake else ) {
        echo 'some other code';
    }
}    
?>