Developing at Full Speed with Xdebug

You won't believe this incredible hack!

Posted on Jul 23, 2018
Tags webdev, server, php, xdebug, docker

A quick history

ed: If you want to jump right to the solution, jump ahead to Nginx map.

Docker for Mac is very slow.

It is so slow that I purchased a new Dell XPS laptop and for the first time in 6 years am now using a non-MacOS (Fedora) machine as my daily driver.

Not everyone has the luxury of switching their OS, though, and they are stuck on slow Docker.

A normal Symfony 2.4 application will commonly see between 400ms-750ms response times in development mode, without Xdebug installed. If Xdebug is activated response times of 1,200ms+ can frustrate even the most devoted Xdebug fan.

Before switching to Fedora I tried everything I could to minimize Xdebug’s impact on performance. I have been using Xdebug for several years and while I always felt the benefits of Xdebug far out-weighed the extra slowness, I knew there had to be a better way.

How Xdebug decides to run

There are several ways of enabling Xdebug for a specific session. The more popular ways are using cookies, like those generated by PhpStorms bookmarklets. You can also kick Xdebug off via CLI to debug command line scripts without a web portion.

All methods share the same requirement: Xdebug must be installed and loaded on the system to work.

The all also let the PHP layer of your application decide whether to enable Xdebug for the current session.

If you take a look at PhpStorm’s bookmarklets, the code is actually quite simple:

javascript:(/** @version 0.5.2 */function() {document.cookie='XDEBUG_SESSION='+'xdebug'+';path=/;';})()

It simply sets a cookie named XDEBUG_SESSION and sets a value to it. By default PhpStorm wants to use… phpstorm, but I always set it to xdebug as above.

Knowing this,would it be possible to move the decision one layer above, out of PHP’s hands? Instead of PHP reading for the XDEBUG_SESSION cookie and acting on it, do it somewhere else.

Docker configuration

A normal Docker PHP application looks like:

  • 1 webserver like Nginx
  • 1 PHP container with Xdebug installed

The problem so far has been that the PHP side of things is slowing everything down, due to Xdebug being installed.

What if instead of the above, we had:

  • 1 webserver like Nginx
  • 1 PHP container with Xdebug installed (named php_xdebug)
  • 1 PHP container without Xdebug installed (named php)

The trick here is making the decision to invoke Xdebug before PHP becomes aware that a new request is being processed. Thankfully, Nginx’s map can help us tremendously!

Nginx map

Nginx maps are quite simple, and what I came up with to handle the Xdebug cookie requirement is this:

map $cookie_XDEBUG_SESSION $my_fastcgi_pass {
    default php;
    xdebug php_xdebug;

In PHP the above would look like this:


switch ($cookie_XDEBUG_SESSION) {
    case 'xdebug':
        $my_fastcgi_pass = 'php_xdebug';
        $my_fastcgi_pass = 'php';

$cookie_XDEBUG_SESSION is the cookie set by PhpStorm’s bookmarklet, and $my_fastcgi_pass is the server Nginx will use for this request.

A full Symfony 3 Nginx config would look like this:

# default Docker DNS server

map $cookie_XDEBUG_SESSION $my_fastcgi_pass {
    default php;
    xdebug php_xdebug;

server {
    listen *:8080 default_server;

    server_name _;
    root /var/www/public;

    autoindex off;

    location / {
        try_files $uri /app.php$is_args$args;

    location ~ ^/(app_dev|config)\.php(/|$) {
        set $path_info $fastcgi_path_info;

        fastcgi_pass $my_fastcgi_pass:9000;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;

        include fastcgi_params;

        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param DOCUMENT_ROOT $realpath_root;

    location ~ ^/app\.php(/|$) {
        set $path_info $fastcgi_path_info;

        fastcgi_pass php:9000;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;

        include fastcgi_params;

        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param DOCUMENT_ROOT $realpath_root;


    location ~ \.php$ {
        return 404;

You can see how we’re actually using the result of the map at fastcgi_pass $my_fastcgi_pass:9000;.

The Nginx part is simple, but critical. Nginx reads the cookie and sends traffic either to the php server or the php_xdebug server. There is nothing magical about this, and PHP is not aware that a request is being made until Nginx has made the decision to route the request to its server. The other server would remain blissfully unaware of the request.

Good solution or hacky workaround?

Your normal workflow with Xdebug would look like this:

  • You enable the Xdebug cookie in your browser
  • You create a breakpoint and enable the listener in your IDE
  • You load the page in your browser, Xdebug and your IDE connect and stop at the breakpoint defined.

With my Nginx map solution, your workflow remains exactly the same. It does not change!

But, instead of Xdebug slowing down every single request, whether you have set a breakpoint or not, whenever you do not have the XDEBUG_SESSION cookie enabled, you are hitting the non-Xdebug server! In real-world testing this means that the majority of my requests are now routed directly to the PHP server that does not have Xdebug installed.

On my MacBook Pro requests see a normal response time of sub-500ms. This is still not great (because, Docker), but is worlds better than every single request being 1,000ms+!

When I do want to enable debugging, I set my breakpoint, enable the XDEBUG_SESSION cookie by using PhpStorm’s bookmarklets, and Nginx routes my request to the PHP server with Xdebug installed.

When this happens, I have a breakpoint enabled and will be looking at my IDE, so response times do not matter at all. The biggest (and only) painpoint that Xdebug introduces is completely eliminated.

What took so long?

Until 6 months ago I was using Vagrant virtual machines for all my development. My FOSS PuPHPet is evidence that I was all-in on VMs.

However, since completely making the switch to Docker containers, my workflow has changed for the better, even with the slowness Docker for Mac shows.

Docker containers have the benefit of being incredibly light-weight compared to virtual machines, and this makes spinning up two PHP containers for each of my projects completely feasible.

This solution would have been much heavier on virtual machines. On containers it is easier, simpler and faster.


I have not found any downsides to this technique. If you use file-based sessions (why?) then you’ll have to share the volumes between the PHP containers.

Test it out

If you want to try this out in a real-world project, and are not familiar enough with Docker to do so quickly, then you can clone my new FOSS Dashtainer repo and run the bin/init script to get you up and running in a single step.

On a very related note, if you want to get started with Docker but need some help for the initial hurdles, my FOSS Dashtainer is the successor to my other FOSS PuPHPet and is aimed at developers who need an introduction to the strange new world of containers.

edit: I have created a barebones repo for you to test this out. Clone my jtreminio/blog repo and run init inside the developing-at-full-speed-with-xdebug directory.

Bonus: Identical configurations in Windows, MacOS and Linux

Docker has a special hostname, host.docker.internal that points to the host machine. Unfortunately this only works on Windows and MacOS, Docker on Linux does not have this feature.

This makes setting the xdebug.remote_host value in the PHP INI annoying becuase not all of your team can simply spin up your Docker container and have Xdebug connect back to the host automatically if they are not on Linux.

There is xdebug.remote_connect_back which does just that, though, except it has always been a double-edged sword because it would attempt to connect back for every single request. My solution above resolves this completely!

Simply set xdebug.remote_connect_back=1 in your INI and Xdebug will connect back to your IDE everytime you enabled the session using PhpStorm’s bookmarklets.

Wrapping it up

With this small technique you gain the ability to develop at full speed without sacrificing modern tools like the incredible Xdebug. Usually with these sorts of things there’s also some downside, but all I see is positives.

Until next time, this is Señor PHP Developer Juan Treminio wishing you adios!