get plugin files code example

Example 1: how to get plugin directory path in wordpress

if ( is_admin() ) {
    include_once( plugin_dir_path( __FILE__ ) . 'includes/admin-functions.php' );
} else {
    include_once( plugin_dir_path( __FILE__ ) . 'includes/front-end-functions.php' );
}

Example 2: how to get plugin directory path in wordpress

/**
 * Include CSS file for MyPlugin.
 */
function myplugin_scripts() {
    wp_register_style( 'foo-styles',  plugin_dir_url( __FILE__ ) . 'assets/foo-styles.css' );
    wp_enqueue_style( 'foo-styles' );
}
add_action( 'wp_enqueue_scripts', 'myplugin_scripts' );

Answer
http://example.com/wp-content/plugins/my-plugin/assets/foo-styles.css