What is the difference between require and require-dev sections in composer.json?

1. According to composer's manual:

   require-dev (root-only)

   Lists packages required for developing this package, or running tests, etc. The dev requirements of the root package are installed by default. Both install or update support the --no-dev option that prevents dev dependencies from being installed.

   So running composer install will also download the development dependencies.

2. The reason is actually quite simple. When contributing to a specific library you may want to run test suites or other develop tools (e.g. symfony). But if you install this library to a project, those development dependencies may not be required: not every project requires a test runner.

Different Environments

Typically, software will run in different environments:

• development
• testing
• staging
• production

Different Dependencies in Different Environments

The dependencies which are declared in the require section of composer.json are typically dependencies which are required for running an application or a package in

• staging
• production

environments, whereas the dependencies declared in the require-dev section are typically dependencies which are required in

• developing
• testing

environments.

For example, in addition to the packages used for actually running an application, packages might be needed for developing the software, such as:

• friendsofphp/php-cs-fixer (to detect and fix coding style issues)
• squizlabs/php_codesniffer (to detect and fix coding style issues)
• phpunit/phpunit (to drive the development using tests)
• etc.

Deployment

Now, in development and testing environments, you would typically run

$ composer install

to install both production and development dependencies.

However, in staging and production environments, you only want to install dependencies which are required for running the application, and as part of the deployment process, you would typically run

$ composer install --no-dev

to install only production dependencies.

Semantics

In other words, the sections

• require
• require-dev

indicate to composer which packages should be installed when you run

$ composer install

or

$ composer install --no-dev

That is all.

Note Development dependencies of packages your application or package depend on will never be installed

For reference, see:

• https://getcomposer.org/doc/04-schema.md#require
• https://getcomposer.org/doc/04-schema.md#require-dev

From the composer site (it's clear enough)

require#

Lists packages required by this package. The package will not be installed unless those requirements can be met.

require-dev (root-only)#

Lists packages required for developing this package, or running tests, etc. The dev requirements of the root package are installed by default. Both install or update support the --no-dev option that prevents dev dependencies from being installed.

Using require-dev in Composer you can declare the dependencies you need for development/testing the project but don't need in production. When you upload the project to your production server (using git) require-dev part would be ignored.

Also check this answer posted by the author and this post as well.