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Drupal Core Development Composer Project

This is a Composer project template for developing Drupal core.

It allows:

  • a clean git clone of Drupal core.
  • Composer dependencies of Drupal core are installed, so Drupal can be installed and run as normal.
  • other Composer packages you might want, such as Drush, Devel module, and Admin Toolbar module, can be installed too, but don't affect the composer files that are part of Drupal core.


Get this into Drupal core! See


To install a Drupal project for working on Drupal core:

$ composer create-project joachim-n/drupal-core-development-project

Composer will clone Drupal core into a 'repos/drupal' directory within the project, and then symlink that into the project when it installs Drupal core.

Drupal core is checked out to its default branch, which is currently 11.x. To start on a different branch without first checking out 11.x, you can use the --no-install option with the composer create-project command, then change the branch of the Drupal core clone, then do composer install.

Once the Composer installation is complete, you can install Drupal as normal, either with drush si or with the web UI.

Installation on DDEV

First, create the folder for your project and cd into it. Then:

$ ddev config --project-type=drupal10 --docroot=web --create-docroot
$ ddev start
$ ddev composer create joachim-n/drupal-core-development-project


Contrib and custom tests

Contrib and custom module tests can't be run. For details, see #14.

'Could not scan for classes' error messages

During some Composer commands you may see multiple copies of this error message:

Could not scan for classes inside [Drupal class filename].

These are harmless and can be ignored.

Developing Drupal core

You can use the Drupal core git clone at 'repos/drupal/' in any way you like: create feature branches, clone from issue forks, and so on. Changes you make to files in the git clone affect the project, since the git clone is symlinked into it.

Managing the Composer project

You can install any Composer packages you like, including Drupal contrib modules, without affecting the git clone of Drupal core. To work with Composer, you need to be in the root directory of the project.

Changes to the git clone's composer.json will be taken into account by Composer. So for example, if pulling from the main branch of Drupal core changes Composer dependencies, and in particular if you change to a different core major or minor branch, you should run composer update on the project to install these.

Running tests

The following are required to run tests.

PHPUnit configuration

The simplest way to run tests with this setup is to put the phpunit.xml file in the project root and then run tests from there:

$ vendor/bin/phpunit web/core/PATH-TO-TEST-FILE/TestFile.php

To set this up, copy Drupal core's sample phpunit.xml file to the project root:

$ cp web/core/phpunit.xml.dist phpunit.xml

Then change the bootstrap attribute so the path is correct:

<phpunit bootstrap="web/core/tests/bootstrap.php"


You can set up debugging in an IDE that's open at the repos/drupal folder, so that it recognises the process being run from the project root.

For example, in VSCode, this is done as follows in the debugger configuration:

"pathMappings": {
  // Make this work with the root project.
  "/ABSOLUTE/PATH/TO/PROJECT/repos/drupal": "${workspaceRoot}"

Technical details

The rest of this document is gory technical details you only need to know if you're working on this project template or debugging it.

How it works

The composer.json at the project root uses a Composer path repository so that when the drupal/drupal package is installed, it's symlinked in from the Drupal core git clone, at the branch that the clone has checked out.

Drupal core itself defines path repositories in its top-level composer.json. These need to be overridden in the project root composer.json so they point to inside the Drupal core git clone.

Additionally, the paths to the drupal/core-recommended and drupal/core-dev packages are defined as path repositories, so that the package versions which are fixed in those metapackages are respected in the project. This means the same versions are installed as if installing Composer packages on a plain git clone of Drupal core.

Manual Installation

Clone the repository for this template into, say, 'drupal-dev'.

$ cd drupal-dev

# Create a folder in which to store git clones, which Composer will symlink in.
$ mkdir repos
$ cd repos

# Clone Drupal core, to whatever branch you like.
$ git clone --branch 9.2.x

# Go back to the project root.
$ cd ..

# Install packages with Composer.
$ composer install

The Drupal core git clone will be clean apart from:


Since it doesn't have a .gitignore at the top level, you can add one to ignore those files if you like.

Project template development installation

To test how Composer creates a new project from the template, you need a git clone of the template repository.

In a separate location, do:

$ composer create-project joachim-n/drupal-core-development-project NEW_PROJECT_DIRECTORY --stability=dev --repository='{"url":"/path/to/git/clone/of/project/template/","type":"vcs"}'


Several workarounds are necessary to make Drupal core work correctly when symlinked into the project. These are all taken care of by Composer scripts during installation. Details are below.

Most if not all of these will no longer be needed once is fixed.

Vendor folder

The vendor folder has to be symlinked into the Drupal core repository, because otherwise code in core that expects to find a Composer autoloader fails.

This is done by a Composer script after initial installation. The manual command is:

ln -s ../../vendor ./repos/drupal/vendor

App root files patches

The index.php and update.php scaffold files have to be patched after they have been copied to web/index.php, because otherwise DrupalKernel guesses the Drupal app root as incorrectly being inside the Drupal core git clone, which means it can't find the settings.php file.

This is done by a Composer script after initial installation. The manual commands are:

cd web && patch -p1 <../scaffold/scaffold-patch-index-php.patch
cd web && patch -p1 <../scaffold/scaffold-patch-update-php.patch

See for more detail.

Drush rebuild command

The Drush cache:rebuild command does not work correctly if contrib modules are present, because it calls drupal_rebuild() which lets DrupalKernel guess the app root incorrectly.

This project template contains a /drush folder which has a command class which replaces that command with custom code to correctly handle the app root.

Simpletest folder

When running browser tests, the initial setup of Drupal in FunctionalTestSetupTrait::prepareEnvironment() creates a site folder using the real file locations with symlinks resolved, thus repos/drupal/sites/simpletest, but during the request to the test site, Drupal looks in /web/sites/simpletest.

Additionally, the HTML files output from Browser tests are written into the Drupal core git clone, and so the URLs shown in PHPUnit output are incorrect.

The fix for both of these is to create the simpletest site folder in the web root and symlink it into the Drupal core git clone.

This is done by a Composer script after initial installation. The manual command is:

mkdir -p web/sites/simpletest
ln -s ../../../web/sites/simpletest repos/drupal/sites

Autoload of Drupal composer testing classes

Drupal's /composer folder is not symlinked and therefore isn't visible to Composer. It's needed for some tests, and so is declared as an autoload location.


Project template for developing Drupal core with a git clone.






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