function test(){
    echo '123';
   die();
}
add_action('wp_ajax_test', 'test');
add_action('wp_ajax_nopriv_test', 'test');

====== AJAX
$('#gift_pillow').on('click',function(e) {
                e.preventDefault();
                console.log(ajaxurl);
                $.ajax({
                    url: ajaxurl,
                    type: 'POST',
                    data: {
                        action:'test'
                    }
                }).done( function (response) {
                    console.log(response);
                    if( response.error != 'undefined' && response.error ){
                        return true;
                    } else {
                        window.location.href = '/cart/';
                    }
                });
            });