Add PHP to WordPress Text Widget

Add this codeblock to your functions.php file:

add_filter('widget_text','execute_php',100);
function execute_php($html){
     if(strpos($html,"<"."?php")!==false){
          ob_start();
          eval("?".">".$html);
          $html=ob_get_contents();
          ob_end_clean();
     }
     return $html;
}

Code courtesy of http://www.emanueleferonato.com/2011/04/11/executing-php-inside-a-wordpress-widget-without-any-plugin/