PHP Composer Cheat Sheet

Explains Composer CLI commands, composer.json structure, semantic version constraints, and PSR-4 autoloading for PHP dependency management.

1 PageBeginnerApr 15, 2026

Composer CLI

Common commands for managing PHP dependencies.

bash

composer init                            # Interactively create composer.jsoncomposer require monolog/monolog          # Add a dependency and install itcomposer require --dev phpunit/phpunit    # Add a dev-only dependencycomposer install                          # Install exact versions from composer.lockcomposer update                           # Update deps within version constraintscomposer dump-autoload                    # Regenerate the autoloadercomposer show                             # List installed packages

composer.json

Core structure of a Composer manifest file.

json

{  "name": "acme/myapp",  "require": {    "php": "^8.2",    "guzzlehttp/guzzle": "^7.8"  },  "require-dev": {    "phpunit/phpunit": "^10.0"  },  "autoload": {    "psr-4": {      "App\\": "src/"    }  }}

Composer Concepts

Version constraints and autoloading fundamentals.

^7.8 (caret)- Allows updates that don't change the leftmost non-zero digit, e.g. >=7.8.0 <8.0.0
~7.8.0 (tilde)- Allows patch-level updates only when three components are given, e.g. >=7.8.0 <7.9.0
composer.lock- Pins the exact resolved versions; commit it for applications to guarantee reproducible installs
require vs require-dev- require-dev packages (tests, linters) are excluded when installing with --no-dev in production
PSR-4 autoloading- Maps namespace prefixes to directories so classes load automatically without manual require statements
vendor/ directory- Where Composer installs packages and generates the vendor/autoload.php bootstrap file

Using the Autoloader

Bootstrapping PSR-4 classes and file-based helpers.

php

// index.phprequire __DIR__ . '/vendor/autoload.php';use App\Models\Post;$post = new Post();// composer.json can also autoload plain files:// "autoload": {//   "psr-4": { "App\\": "src/" },//   "files": ["src/helpers.php"]// }

Pro Tip

Never hand-edit composer.lock without running composer install afterward to verify the resolved set actually installs cleanly — a stale or manually edited lock file can silently drift from composer.json.

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