What is the easiest way to include bootstrap in Craft? How is the easiest way to include bootstrap to Craft? I've seen there's already a bootstrap.php in the "app" folder... what can I do with this guy?
There are other ways, but since you've asked for the easiest, that's how I'd do it...
Just add it to /templates/_layout.twig and extend it to other templates: like /templates/index.twig starts with {% extends &quot;_layout&quot; %}
&lt;!doctype html&gt;
&lt;html&gt;
&lt;head&gt;
    ...
    &lt;link rel=&quot;stylesheet&quot; href=&quot;/css/bootstrap.css&quot;&gt;
    ...
&lt;/head&gt;
&lt;body&gt;
&lt;div class=&quot;container&quot;&gt;
    &lt;div class=&quot;row&quot;&gt;
        &lt;div class=&quot;col&quot;&gt;
            {% block content %}{% endblock %}
        &lt;/div&gt;
    &lt;/div&gt;
&lt;/div&gt;
// don't forget jquery/popper here
&lt;script src=&quot;/js/jquery-3.3.1.min.js&quot;&gt;&lt;/script&gt;
&lt;script src=&quot;/js/popper.min.js&quot;&gt;&lt;/script&gt;
&lt;script src=&quot;/js/bootstrap.js&quot;&gt;&lt;/script&gt;
&lt;/body&gt;
&lt;/html&gt;

PS. Make sure `bootstrap.css` and `bootstrap.js` are placed in `/web/css` and `/web/js`