Debugging PHP

There are many ways to go about debugging a PHP application, and one of the most effective ways is using a debugger. One of the most powerful tools in the PHP community to go about doing this is Xdebug.

What is Xdebug?

Xdebug enables you to do the following:

  • debug and profile PHP applications and scripts
  • interactively debug running code
  • measure the performance of your application
  • see the state of your application at a point in time

Xdebug gives you all sorts of visibility into the internals of your application, like what variable values are at a certain point in time, what functions are taking a long time to execute, as well as what the return values of functions are. It gives you the ability to step through the execution of your application function by function, or even line by line if you really want to.


Trellis is configured with Xdebug and ready to rock out of the box in development. All you have to do is select a compatible debugger. Xdebug is designed to be used with a DBGP-compatible debugger in order to interface with Xdebug on your site. PHPStorm comes with support for this out of the box and Sublime Text, Visual Studio Code, and Vim have plugins available.


The variables used in the roles/xdebug role directly correlate to the configuration options used by Xdebug itself. For example, Xdebug has the option xdebug.scream to disable PHP error suppression using the @ symbol before function calls. The corresponding Trellis variable would be xdebug_scream.

You can see all the available configuration options in roles/xdebug/defaults/main.yml and read about how they're used in Xdebug on their documentation page. Trellis ships with pretty sane defaults, but this gives you the option to override if necessary. To change those variables, it's recommended you set them in group_vars/<environment>/php.yml.

Using Xdebug in production

While we default to installing Xdebug in development, installing it in any other environment is "opt-in." It is not recommended to use Xdebug in production, but it can be extremely useful in debugging production-like environments. For example, if there's an issue you're encountering in Production, but cannot reproduce in Development (aka, your Vagrant environment), it's likely the problem lies with something specific to your VPS provider. Duplicating your production environment and sanitizing the data using something like WP Hammer will allow you to debug your production environmment without affecting it. This is where bin/ comes in.

bin/ Xdebug + SSH tunnels

Xdebug gives a lot of visibility into your application that you do not want to give to anyone. Because of this, you want to restrict access to who is allowed to initiate a debugging session. The way we go about doing that is by creating a remote SSH tunnel from the VPS to your local computer. bin/ makes it trivial to set up the connection by installing Xdebug if it is not already on the remote host as well as establishing the SSH tunnel between your server and your computer.

By default, Trellis configures Xdebug to look for a debugging session on the server's localhost port 9000:

# roles/xdebug/defaults/main.yml
xdebug_remote_host: localhost
xdebug_remote_port: 9000

Because your debugger is located on your computer and not the server, Xdebug would attempt to communicate with localhost:9000 unsuccessfully and proceed with the request as normal. Using bin/ creates a tunnel from the server's localhost:9000 to your computer's localhost:9000, bridging the gap and allowing the two to communicate.

Establishing the tunnel

First, let's look at the command we'll be using to create the tunnel:

./bin/ <action> <host>

The argument action can be open or close and host is the hostname, IP, or inventory alias in your hosts/<environment> file.

Provided this hosts file:

# let's pretend hosts/staging

some_inventory_hostname ansible_ssh_host=



You would execute:

./bin/ open some_inventory_hostname

This script runs the xdebug-tunnel.yml playbook with the necessary variables to install Xdebug on the environment as well as establish the tunnel.

To close the tunnel, as well as disable Xdebug, run:

./bin/ close some_inventory_hostname

This will remove the /etc/php/7.0/fpm/conf.d/20-xdebug.ini symlink, effectively disabling it for that environment while leaving xdebug installed. It also closes the SSH connection.

If you don't use inventory aliases in your host files, you can also use an ip address directly instead of the alias. For example, if your hosts file looks like this:



You can do this:

./bin/ open

You must specify the host exactly the same when opening and closing the tunnel. It would cause an error to open the tunnel with a host of some_inventory_hostname then close with a host of This is because the tunnel socket is created using the host parameter you pass:

/tmp/trellis-xdebug-{{ provided host }}

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