Custom ‘Read More’ button using <a class=”button” with php

Here’s what I’m attempting to do. I’d like to have post excerpts with a limited number of words, with a custom ‘read more’ button.

Currently I’m using the following to set a limited number of words for my excerpts.

<?php if ($post->post_excerpt != "" ) {the_excerpt();}else {the_content_rss('', FALSE, '', 40);}?>

Is it possible to include with php script to make each post link to it’s respective full-post page?