PSR-4 directory mapping in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is PSR-4 in PHP?

PSR-4 is a standard for autoloading PHP classes from file paths that match their namespaces. It helps PHP find and load classes automatically based on their namespace and class name.

beginner

How does PSR-4 map namespaces to directories?

PSR-4 maps a namespace prefix to a base directory. The namespace parts after the prefix become subdirectories inside that base directory, and the class name becomes the file name with a .php extension.

beginner

Example: If the namespace prefix is App\ mapped to src/, where is the class App\Controllers\HomeController located?

The class App\Controllers\HomeController is located at src/Controllers/HomeController.php.

intermediate

Why is PSR-4 preferred over older autoloading methods?

PSR-4 is preferred because it is simple, standardized, and avoids conflicts. It makes code easier to organize and share by following a clear rule to find classes based on namespaces.

intermediate

What happens if a class file does not follow PSR-4 directory mapping?

If a class file does not follow PSR-4 mapping, the autoloader will not find the file automatically, causing errors like "class not found". You must then load the file manually or fix the directory structure.

What does PSR-4 autoloading map to find a class file?

A Function name to URL path

B Class name to database table

C Namespace prefix to base directory

D Variable name to environment variable

PSR-4 maps a namespace prefix to a base directory to locate class files.

If the namespace is MyApp\Models\User and prefix MyApp\ maps to lib/, where is the class file?

A lib/User.php

B lib/MyApp/Models/User.php

C Models/User.php

D lib/Models/User.php

The namespace after prefix becomes subdirectories: Models/User.php inside lib/.

What file extension does PSR-4 expect for class files?

A .php

B .txt

C .inc

D .class

PSR-4 expects PHP class files to have the .php extension.

What error occurs if PSR-4 mapping is incorrect?

A Class not found error

B Syntax error

C Memory overflow

D Division by zero

Incorrect mapping causes "class not found" errors because the autoloader can't find the file.

Which of these is NOT a benefit of PSR-4?

A Standardized autoloading

B Automatically compiles PHP to machine code

C Simplifies class loading

D Avoids naming conflicts

PSR-4 does not compile PHP to machine code; it only standardizes autoloading.

Explain how PSR-4 directory mapping works in PHP autoloading.

Think about how namespaces relate to folder paths.

Describe a common error when PSR-4 directory mapping is not followed and how to fix it.

Consider what happens if the autoloader can't find the file.