Get URL of a Plugin File

If you want to get the URL of a plugin file (e.g. a CSS file), use the snippet below:

plugins_url( 'css/style.css', __FILE__ );

To include the CSS file, add this:

add_action( 'wp_enqueue_scripts', 'wpkb_add_custom_stylesheet' );
function wpkb_add_custom_stylesheet() {
    wp_enqueue_style( 'my-stylesheet', plugins_url( 'css/style.css', __FILE__ ), array() );
}