composer.json

composer.json is the heart of a Composer project. It is a JSON file in your project’s root folder that lists what your project needs.

Create one

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composer init

Composer asks a few questions (name, description, author, packages) and writes the file. You can also create the file by hand.

A realistic example

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{
    "name": "acme/blog-app",
    "description": "A small blog application",
    "type": "project",
    "require": {
        "php": "^8.2",
        "league/commonmark": "^2.0"
    },
    "require-dev": {
        "phpunit/phpunit": "^11.0"
    },
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\": "tests/"
        }
    },
    "scripts": {
        "test": "phpunit"
    },
    "config": {
        "sort-packages": true
    }
}

The main fields

require and the PHP version

Putting "php": "^8.2" in require tells Composer that your project needs PHP 8.2 or newer. Composer then refuses to install packages that do not work on that version, and warns you if you run it on an older PHP.

Version constraints

"^2.0" is a constraint: any version from 2.0.0 up to (but not including) 3.0.0. Managing Dependencies explains them.

The file must be valid JSON

JSON is strict: double quotes only, no trailing commas, no comments. Check a file with:

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composer validate

Backslashes in JSON

In the autoload section, the namespace ends with a backslash. In JSON, a backslash must be written twice: "App\\".

Commit or ignore?