composerjson

composer.json is a manifest file used by Composer, the dependency manager for PHP. It describes a project's dependencies, autoloading rules, scripts, and metadata, and is read by Composer to install and update packages. The file sits at the project root and is written in JSON, requiring valid JSON syntax with no comments.

Common sections in a composer.json include require for runtime dependencies and require-dev for development dependencies; autoload and autoload-dev to define autoloading rules (such as PSR-4, PSR-0, classmap, or files); and scripts for event hooks like post-install or pre-update. Additional fields can specify metadata such as name, description, license, authors, keywords, type, homepage, and repositories to override package sources. The minimum-stability and prefer-stable fields influence which versions Composer will consider acceptable.

Version constraints in composer.json use semantic versioning. Examples include ^1.2.3 to allow compatible updates within the 1.x range, ~1.2.0 for updates up to but not including 1.3.0, and >=1.0.0 for explicit lower bounds. The file may also declare PHP requirements and platform constraints.

When running Composer, Composer reads composer.json to resolve dependencies and then generates or updates composer.lock to lock exact package versions. Running composer install installs according to the lock file, ensuring reproducible installations, while composer update updates dependencies and refreshes the lock file. Autoloading configurations map namespaces to directories or files, enabling automatic class loading at runtime.

Validation is supported by commands like composer validate, which checks syntax and the viability of the defined constraints. Keeping composer.json under version control helps preserve project configuration, while the accompanying lock file ensures consistent environments across installations.