Method 2: Customize WordPress widget title.Full tutorial: http://www.rvamedia.com/wordpress/customize-wordpress-widget-title

widget-title-2.php

<?php
//* Do NOT include this comment or the opening php tag above
//* Insert SPAN tag into widgettitle
add_filter( 'dynamic_sidebar_params', 'b3m_wrap_widget_titles', 20 );
function b3m_wrap_widget_titles( array $params ) {
        
        // $params will ordinarily be an array of 2 elements, we're only interested in the first element
        $widget =& $params[0];
        $widget['before_title'] = '<h4 class="widgettitle"><span class="sidebar-title">';
        $widget['after_title'] = '</span></h4>';
        
        return $params;
        
}