If/Else in Modx

I'm trying to wrap in my template a block in conditionals:

<div class="widget widget_categories trip_widget">
    <div class="kd-widget-title"><h4>Book online</h4></div>
    <div class="trip-book-button">
        <a href="[(site_url)]book-online.html/#id=[*trip-slug*]">Book now online</a>        
    </div>
</div>

I need to show that block only if [*trip-slug*] is present.

I've read through the forum and the documentation but I can't seem to get the syntax right.

What am I missing?

In Evolution you need the PHx plugin (https://github.com/Temus/PHx) to process resource or template variable tags. With that plugin you could use the following code:

[*trip-slug:ne=`
  <div class="widget widget_categories trip_widget">
    <div class="kd-widget-title"><h4>Book online</h4></div>
    <div class="trip-book-button">
      <a href="[(site_url)]book-online.html/#id=[*trip-slug*]">Book now online</a>        
    </div>
  </div>
`*]

Placeholder tags inside Ditto are parsed with a (limited) PHx parser and you could use the following inside a Ditto template without the plugin:

[+trip-slug:ne=`
  <div class="widget widget_categories trip_widget">
    <div class="kd-widget-title"><h4>Book online</h4></div>
    <div class="trip-book-button">
      <a href="[(site_url)]book-online.html/#id=[+trip-slug+]">Book now online</a>        
    </div>
  </div>
`+]

In both cases you could also work with the Evolution core if Snippet.

[[if? &is=`[*trip-slug*]:not_empty` &then=`
  <div class="widget widget_categories trip_widget">
    <div class="kd-widget-title"><h4>Book online</h4></div>
    <div class="trip-book-button">
      <a href="[(site_url)]book-online.html/#id=[*trip-slug*]">Book now online</a>        
    </div>
  </div>
`]]

Replace the [* *] with [+ +] inside of Ditto template chunks.