Slim 4 - HTMX Server-side Integration

Daniel Opitz
Daniel Opitz
12 Aug 2023


In the evolving landscape of web development, HTMX (HTML extensions for modern web applications) emerged as a compelling tool for those seeking to enhance their web applications without relying on heavy JavaScript frameworks. At its core, HTMX allows developers to access modern browser features like AJAX, WebSockets, and Server-Sent Events directly from HTML, without needing to write any JavaScript. This paves the way for creating dynamic and interactive web applications with a focus on progressive enhancement and simplicity.

One of the most striking features of HTMX is its capacity to swap parts of the webpage with updated content from the server, making it easy to build applications that feel responsive and fast. By leaning on standard HTTP requests and responses, developers can update sections of a page in reaction to user interactions, all while writing minimal code. This approach keeps web pages lightweight and accessible.

In essence, HTMX is a bridge between the simplicity of static HTML and the dynamic capabilities of modern web apps. It offers an elegant alternative to the traditional, JavaScript-heavy approaches, making it an excellent choice for developers prioritizing performance, accessibility, and maintainability.

If you’re diving into HTMX, you’ll find that it fits seamlessly with various backend frameworks (such as Slim Framework) and architectures, offering a rich toolset for building modern web interfaces.

In this article, I want to show an approach to integrate HTMX with any template engine, such as Twig, Plates or the Slim PHP View component.

Include HTMX

Add the HTMX library to your HTML. You can either download and serve it locally or include it from a CDN:

<script src=""></script>

Suppose you want a button that, when clicked, fetches content from a Slim route and replaces the content of a div.

In your template, you can have something like:

<div id="dynamicContent">
  <!-- This content will be replaced by the server response -->

<button hx-get="/fetch-content" hx-target="#dynamicContent">
  Fetch Content

Define the Slim Route

In your Slim routes file, define the route that the button will fetch from:

$app->get('/fetch-content', function ($request, $response) {
    // Generate or fetch the new content
    $newContent = 'This is the new content!';
    // Return the new content

    return $response;

Note that HTMX will not automatically add the base URL that to the HTTP request.

So when you are running your application within a sub-directory of the webservers document-root, then you should also define the full URL path in the hx-get value, for example hx-get="/my-app/fetch-content"

Detecting HTMX requests

If you want to optimize further, you can detect if a request is coming from HTMX by checking the headers. HTMX adds a special header HX-Request with the value true. This can be useful if you want to render partial views (without the full layout) for HTMX requests:

$app->get('/fetch-content', function ($request, $response) {
    $isHtmx = $request->getHeaderLine('HX-Request') === 'true';
    if ($isHtmx) {
        // Return a partial view or just some content
        $response->getBody()->write('partial view or just some content');
    } else {
        // Return the full view
        $response->getBody()->write('full view or just some content');

    return response;

This is a simple integration, but it demonstrates the power of HTMX.

By combining Slim’s routing and view rendering with dynamic capabilities of HTMX, you can create rich and interactive web applications without heavy frontend frameworks. Adjust and expand as needed for your application’s requirements!