How to get plugin URL?
echo plugin_dir_url( __FILE__ ) . 'assets/foo-styles.css'; # OUTPUT WILL BE http://example.com/wp-content/plugins/my-plugin/assets/foo-styles.css
Refferences : https://codex.wordpress.org/Function_Reference/plugin_dir_url
echo plugin_dir_url( __FILE__ ) . 'assets/foo-styles.css'; # OUTPUT WILL BE http://example.com/wp-content/plugins/my-plugin/assets/foo-styles.css
Refferences : https://codex.wordpress.org/Function_Reference/plugin_dir_url