Using echo or return in the_content filter

Solution:

the_content() requires a simple $content to be returned…otherwise it will strip tags. If you want to return the content, you should be able to to so via:

add_filter('the_content', 'my_funct');

function my_funct($content) {
    $content .= '<a href="www.mysite.com">Link</a>';
    return $content;
}