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docs/xdebug.md

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+# Using Xdebug
+
+The default development image is shipped with [Xdebug](https://xdebug.org/),
+a popular debugger and profiler for PHP.
+
+Because it has a significant performance overhead, the step-by-step debugger is disabled by default.
+It can be enabled by setting the `XDEBUG_MODE` environment variable to `debug`.
+
+On Linux and Mac:
+
+```
+XDEBUG_MODE=debug docker compose up -d
+```
+
+On Windows:
+
+```
+set XDEBUG_MODE=debug&& docker compose up -d&set XDEBUG_MODE=
+```
+
+## Debugging with Xdebug and PHPStorm
+
+First, [create a PHP debug remote server configuration](https://www.jetbrains.com/help/phpstorm/creating-a-php-debug-server-configuration.html):
+
+1. In the `Settings/Preferences` dialog, go to `PHP | Servers`
+2. Create a new server:
+   * Name: `symfony` (or whatever you want to use for the variable `PHP_IDE_CONFIG`)
+   * Host: `localhost` (or the one defined using the `SERVER_NAME` environment variable)
+   * Port: `443`
+   * Debugger: `Xdebug`
+   * Check `Use path mappings`
+   * Absolute path on the server: `/srv/app`
+
+You can now use the debugger!
+
+1. In PHPStorm, open the `Run` menu and click on `Start Listening for PHP Debug Connections`
+2. Add the `XDEBUG_SESSION=PHPSTORM` query parameter to the URL of the page you want to debug, or use [other available triggers](https://xdebug.org/docs/step_debug#activate_debugger)
+
+    Alternatively, you can use [the **Xdebug extension**](https://xdebug.org/docs/step_debug#browser-extensions) for your preferred web browser. 
+
+3. On command line, we might need to tell PHPStorm which [path mapping configuration](https://www.jetbrains.com/help/phpstorm/zero-configuration-debugging-cli.html#configure-path-mappings) should be used, set the value of the PHP_IDE_CONFIG environment variable to `serverName=symfony`, where `symfony` is the name of the debug server configured higher.
+
+    Example:
+
+    ```console
+    XDEBUG_SESSION=1 PHP_IDE_CONFIG="serverName=symfony" php bin/console ...
+    ```
+
+## Troubleshooting
+
+Inspect the installation with the following command. The Xdebug version should be displayed.
+
+```console
+$ docker compose exec php php --version
+
+PHP ...
+    with Xdebug v3.x.x ...
+```