wordpress get plugin root directory

//current path: /home/user/var/www/wordpress/wp-content/plugins/my-plugin/
$dir = plugin_dir_path( __DIR__ );
//$dir is set to /home/user/var/www/wordpress/wp-content/plugins/

function enqueue_scripts() {
    wp_enqueue_script( 'custom-js', plugin_dir_url( __FILE__ ) . 'js/custom.js', array( 'jquery' ), '', true );
    wp_enqueue_style( 'style-css', plugin_dir_url( __FILE__ ) . 'css/style.css' );
}
add_action( 'wp_enqueue_scripts', 'enqueue_scripts');
 
function admin_enqueue_scripts() {
    wp_enqueue_script( 'custom-js', plugin_dir_url( __FILE__ ) . 'js/custom.js', array( 'jquery' ), '', true );
    wp_enqueue_style( 'style-css', plugin_dir_url( __FILE__ ) . 'css/style.css' );
}
add_action( 'admin_enqueue_scripts', 'admin_enqueue_scripts');

function plugin_dir_path( $file ) {
    return trailingslashit( dirname( $file ) );
}