CODE EXAMPLE FOR PHP

wordpress plugin add stylesheet

add_action('wp_enqueue_scripts','register_my_scripts');

function register_my_scripts(){
    wp_enqueue_style( 'style1', plugins_url( 'css/style1.css' , __FILE__ ) );
    wp_enqueue_style( 'style2', plugins_url( 'css/style2.css' , __FILE__ ) );
}