Composer Packages 📦

# Composer is PHP's dependency manager. It downloads the libraries
# your project needs, and writes a tiny PHP file that loads them all.

# 1) Start a brand-new project. This asks a few questions, then
#    writes a composer.json describing your project.
composer init

# 2) Add a dependency. This downloads the package into vendor/,
#    records it in composer.json, and pins the exact version in composer.lock.
composer require monolog/monolog

# 3) On a fresh checkout (e.g. a teammate clones the repo),
#    install grabs EXACTLY the versions listed in composer.lock.
composer install

# 4) update re-resolves to the newest versions your constraints allow
#    and REWRITES composer.lock with what it picked.
composer update

# composer require monolog/monolog
Using version ^3.7 for monolog/monolog
./composer.json has been updated
Running composer update monolog/monolog
Loading composer repositories with package information
Updating dependencies
Lock file operations: 2 installs, 0 updates, 0 removals
  - Locking monolog/monolog (3.7.0)
  - Locking psr/log (3.0.2)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 2 installs, 0 updates, 0 removals
  - Installing psr/log (3.0.2)
  - Installing monolog/monolog (3.7.0)
Generating autoload files

// composer.json  —  what you WANT (a wish list with version ranges)
{
    "name": "acme/blog",
    "type": "project",
    "require": {
        "php": ">=8.1",
        "monolog/monolog": "^3.7"
    },
    "require-dev": {
        "phpunit/phpunit": "^11.0"
    }
}

// composer.lock  —  what you GOT (a receipt with EXACT versions)
//   Generated automatically. You never edit it by hand.
//   It records the precise version of every package AND every
//   sub-dependency, so every machine installs the identical tree.
{
    "packages": [
        { "name": "monolog/monolog", "version": "3.7.0" },
        { "name": "psr/log",         "version": "3.0.2" }
    ]
}

<?php
// PSR-4 is the standard that maps a NAMESPACE to a FOLDER, so Composer
// can find a class file from its name alone — no require statements.
//
// In composer.json you declare the mapping:
//   "autoload": { "psr-4": { "Acme\\Blog\\": "src/" } }
//
// That single line means:
//   prefix  Acme\Blog\   ->  the  src/  directory
//
//   Acme\Blog\Post            ->  src/Post.php
//   Acme\Blog\Model\Comment  ->  src/Model/Comment.php   (sub-namespace = sub-folder)

// --- src/Post.php ---
namespace Acme\Blog;          // namespace MUST match the folder under the prefix

class Post {
    public function __construct(public string $title) {}
    public function slug(): string {
        return strtolower(str_replace(' ', '-', $this->title));
    }
}

// --- index.php (your app entry point) ---
require 'vendor/autoload.php';   // ONE require — Composer wires up every class

use Acme\Blog\Post;            // pull the class in by its full name

$post = new Post('Hello Composer World');
echo $post->slug() . "\n";      // Composer auto-loads src/Post.php on first use

hello-composer-world

<?php
// Semantic Versioning (SemVer) is MAJOR.MINOR.PATCH, e.g. 3.7.2
//   PATCH (3.7.2 -> 3.7.3)  bug fix,    backward compatible
//   MINOR (3.7.0 -> 3.8.0)  new feature, backward compatible
//   MAJOR (3.x   -> 4.0.0)  BREAKING change, may need code changes
//
// In composer.json you don't pin one version — you give a RANGE
// so you keep getting safe fixes without surprise breakages.

$constraints = [
    // ^ (caret): allow everything up to the next MAJOR — the common default
    '^3.7.0'  => '>=3.7.0  and  <4.0.0',   // gets 3.8, 3.9, 3.10 ... not 4.0
    // ~ (tilde): allow up to the next MINOR — more cautious
    '~3.7.0'  => '>=3.7.0  and  <3.8.0',   // patches only: 3.7.1, 3.7.2 ...
    '~3.7'    => '>=3.7.0  and  <4.0.0',   // (two parts) up to next major
    '3.7.*'   => '>=3.7.0  and  <3.8.0',   // wildcard, same as ~3.7.0
];

foreach ($constraints as $written => $means) {
    echo str_pad($written, 9) . " means  " . $means . "\n";
}

^3.7.0    means  >=3.7.0  and  <4.0.0
~3.7.0    means  >=3.7.0  and  <3.8.0
~3.7      means  >=3.7.0  and  <4.0.0
3.7.*     means  >=3.7.0  and  <3.8.0

// composer.json can define SCRIPTS: named shortcuts for commands you
// run all the time. They live under the "scripts" key.
{
    "require-dev": {
        "phpunit/phpunit": "^11.0",
        "phpstan/phpstan": "^1.12"
    },
    "scripts": {
        "test":  "phpunit",
        "stan":  "phpstan analyse src --level=6",
        "check": [ "@stan", "@test" ]
    }
}

// Run them from the terminal — Composer finds the tool inside vendor/bin:
//   composer test     ->  runs phpunit
//   composer stan     ->  runs phpstan
//   composer check    ->  runs BOTH (the @-prefix calls another script)

# 🎯 YOUR TURN — fill in each blank marked ___ , then run it.
# You're starting a project and want the Guzzle HTTP client library.

# 1) The Composer subcommand that ADDS a dependency
composer ___ guzzlehttp/guzzle        # 👉 the verb that means "I require this"

# 2) After cloning a repo, the subcommand that installs the
#    EXACT versions already pinned in composer.lock
composer ___                          # 👉 not "update" — the one that respects the lock

# ✅ Expected: line 1 downloads guzzlehttp/guzzle into vendor/ ;
#    line 2 installs the locked versions for everyone on the team.

composer require guzzlehttp/guzzle
composer install

<?php
// 🎯 YOUR TURN — a PSR-4 class needs the right namespace and the autoloader.
// The class file is  src/Money/Price.php , and composer.json maps:
//   "autoload": { "psr-4": { "Shop\\": "src/" } }

// 1) Give the class the namespace that matches its folder under the prefix.
//    File is src/Money/Price.php and the prefix Shop\ maps to src/ ...
___ Shop\Money;            // 👉 the keyword that declares a namespace + Shop\Money

class Price {
    public function __construct(public int $cents) {}
    public function pounds(): string { return '£' . number_format($this->cents / 100, 2); }
}

// 2) In index.php, load every Composer class with ONE line.
require '___';             // 👉 the file Composer generates: vendor/autoload.php

use Shop\Money\Price;
echo (new Price(2599))->pounds() . "\n";

// ✅ Expected output:
//    £25.99

£25.99