Can a plugin add to header/footer/body content?

I am just wrapping up my first WordPress plugin and the last thing I would like to do is take a piece of data that the plugin knows about (from wp_options table) and display it in either the header or footer of the site. It will simply be a small styled div with a 6 character code in it.

First of all, is this even possible? Does a plugin have the ability to modify a users theme?

If this is possible, what are some best practices and/or “gotcha’s” to keep in mind when doing this? Obviously all themes are quite different so I want to approach this in the most generic and unobtrusive way possible.

If a plugin cannot do such a thing, I would have to imagine that it can write HTML somewhere when content is rendered, again what are best practices for handling this sort of thing?

This is my first stack exchange post without any code but since this is generic question and not specific to any particular method (yet) I feel that it is still appropriate.

Thanks!

Hooks are going to be your best friend here.

You can filter post content by using the the_content filter for example:

add_filter('the_content', 'wse_174099_append_to_content');

function wse_174099_append_to_content( $content ) {
    //get your data
    $custom_items = get_option( 'option_name' );

    $content .= wpautop( $custom_items );

   //always return when using a filter
    return $content;
}

You can hook into the footer of the site by hooking into the wp_footer action:

add_action('wp_footer', 'your_function');
function your_function() {
    $custom_items = get_option( 'option_name' );
    echo $custom_items;
}

You could also look at the get_header and get_footer actions. The only issue is that you can’t control where the theme calls the header and footer files in – it may be a bad spot to output your code.

Hope this helps!