PSR-4 directory mapping in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

PSR-4 Namespace to Directory Mapping Output

Given the following PSR-4 autoload configuration and class usage, what will be the output when the script runs?

PHP

<?php
// composer.json autoload section
// "autoload": {
//     "psr-4": { "App\\": "src/" }
// }

// File: src/Utils/Helper.php
namespace App\Utils;

class Helper {
    public static function greet() {
        return "Hello from Helper!";
    }
}

// File: test.php
require 'vendor/autoload.php';

use App\Utils\Helper;

echo Helper::greet();

A Fatal error: Class 'App\Utils\Helper' not found

B Hello from Helper!\n

C Parse error: syntax error, unexpected token

D Hello from Helper!

💡 Hint

Remember PSR-4 maps namespaces to directories, so the class file must be in the correct folder matching the namespace.

The PSR-4 autoload maps the namespace 'App\\' to the 'src/' directory. The class Helper is in namespace 'App\\Utils', so it should be in 'src/Utils/Helper.php'. The autoloader loads the class correctly, so calling Helper::greet() outputs 'Hello from Helper!'.

🧠 Conceptual

intermediate

PSR-4 Namespace Prefix and Directory Mapping

In PSR-4 autoloading, if the namespace prefix is "Vendor\\Package\\" mapped to the directory "lib/", where should the class Vendor\Package\Sub\MyClass be located?

A lib/Vendor/Package/Sub/MyClass.php

B lib/MyClass.php

C lib/Sub/MyClass.php

D lib/Sub/MyClass.class.php

💡 Hint

PSR-4 removes the namespace prefix and replaces backslashes with directory separators inside the mapped directory.

PSR-4 autoloading removes the namespace prefix 'Vendor\\Package\\' and maps the rest 'Sub\\MyClass' to 'Sub/MyClass.php' inside the 'lib/' directory.

🔧 Debug

advanced

Why does this PSR-4 autoload succeed in finding the class?

Given this composer.json autoload section and file structure, why does the autoloader succeed to load the class Acme\Tools\Widget? composer.json snippet: { "autoload": { "psr-4": { "Acme\\": "src/Acme/" } } } File structure: - src/ - Acme/ - Tools/ - Widget.php Widget.php content:

A The namespace prefix 'Acme\\' maps to 'src/Acme/', so the class file should be at 'src/Acme/Tools/Widget.php', which is correct, so no error.

B The autoloader requires the class file to be named 'Widget.class.php' instead of 'Widget.php'.

C The class Widget is missing a leading backslash in the namespace declaration.

D The namespace prefix 'Acme\\' should map to 'src/', not 'src/Acme/', so the autoloader looks for 'src/Acme/Acme/Tools/Widget.php' which does not exist.

💡 Hint

Check how the namespace prefix maps to the directory and how the rest of the namespace is appended.

The prefix 'Acme\\' maps to 'src/Acme/', so for the class 'Acme\Tools\Widget' the autoloader strips 'Acme\\' leaving 'Tools\Widget', which becomes 'Tools/Widget.php' appended to 'src/Acme/' resulting in 'src/Acme/Tools/Widget.php'. This exactly matches the file structure, so the class loads successfully.

📝 Syntax

advanced

Identify the PSR-4 compliant namespace and directory mapping

Which of the following composer.json autoload psr-4 mappings is correctly set up for the namespace MyApp\Core\ with classes stored in app/core/ directory?

A "MyApp\\Core": "app/core/"

B "MyApp\\Core\\": "app/core/"

C "MyApp\\Core\\": "app/core"

D "MyApp\\Core\\": "app\\core\\"

💡 Hint

PSR-4 requires the namespace prefix to end with double backslashes and directory path to end with a slash.

Option B correctly uses double backslashes at the end of the namespace prefix and ends the directory path with a slash. Option B misses the trailing backslashes in the namespace prefix. Option B misses the trailing slash in the directory path. Option B uses backslashes in the directory path which is invalid in JSON and for PSR-4.

🚀 Application

expert

Determine the number of classes autoloaded by PSR-4

Given this composer.json autoload section and directory structure, how many classes will be autoloaded correctly by PSR-4? composer.json snippet: { "autoload": { "psr-4": { "Lib\\": "lib/", "Lib\\Utils\\": "lib/utils/" } } } Directory structure: - lib/ - Foo.php (namespace Lib; class Foo) - Bar.php (namespace Lib; class Bar) - utils/ - Helper.php (namespace Lib\Utils; class Helper) - Extra.php (namespace Lib\Utils; class Extra) - Old.php (namespace Lib; class Old) Which classes will be autoloaded correctly?

A Foo, Bar, Helper, and Extra only

B Helper, Extra, and Old only

C Foo, Bar, Helper, Extra, and Old

D Foo, Bar, and Old only

💡 Hint

PSR-4 uses the longest matching namespace prefix to map classes to directories.

The prefix 'Lib\\Utils\\' maps to 'lib/utils/', so Helper and Extra are loaded from there. The prefix 'Lib\\' maps to 'lib/', so Foo and Bar are loaded from 'lib/'. The class Old is in namespace 'Lib' but located in 'lib/utils/Old.php' which does not match the 'Lib\\' prefix directory, so it will not be autoloaded correctly.