Question

How can I changing/ adding code of the Themes with a wordpress plugin? Example: Adding before or after the comments-forms a few lines.

Answer 1

You will need action hooks in your themes. These hooks allow you to add content where the hooks are locating via a plugin, or a child theme.

Thesis and Thematic are WordPress themes with action hooks. Although it’s easy to add action hooks to your own theme.

In the theme, add do_action('your_hook_name'). For example, in the comments.php file:

do_action('after_comments_form');

Now, in your plugin/child theme, add, for example,

add_action('after_comments_form', 'commenting_rules');

. This means we're adding the commenting_rules() function to the hook after_comments_form.

The commenting_rules() function could look like:

function commenting_rules(){
    echo "No spam or similar, read our <a href=\"" . bloginfo('url') . "\"/commenting-rules\">Commenting Rules</a>";
}

Be sure you add the add_action() function below the commenting_rules() function.

Themeshaper (the creator of the Thematic) has a great article on that. Nathan Rice has also written a post explaining action hooks.