The default text widget in WordPress only supports HTML. You can extend the functionality of the text widget by making it possible to execute PHP.
Put this code in the functions.php of the theme you are using:
add_filter('widget_text', 'php_text', 99); function php_text($text) { if (strpos($text, '<' . '?') !== false) { ob_start(); eval('?' . '>' . $text); $text = ob_get_contents(); ob_end_clean(); } return $text; }
Always backup your functions.php before changing it!