


What are the different autoloading strategies supported by Composer (PSR-4, classmap, files)?
Composer's automatic loading strategies are mainly three: PSR-4, classmap and files. 1. PSR-4 is suitable for modern PHP projects that follow namespace specifications. It automatically loads by mapping namespaces to directories, supports subdirectories and does not need to list files manually; 2. classmap is used to load classes that do not comply with PSR-4 naming specifications, such as legacy code or multi-class files. Composer will scan the mapping table of the specified directory to generate class names to paths, and needs to run composer dump-autoload after adding or renaming the class; 3. files are used to explicitly include PHP files that need to be loaded unconditionally at each request, suitable for defining global functions, constants or startup logic, but excessive use may affect performance. These three strategies can exist in the same composer.json at the same time. After modification, composer dump-autoload needs to be executed to update the automatic loading file. In summary, the new code is recommended to use PSR-4, the old code or non-standard named classes use classmap, and the global functions and initialization logic use files.
Composer supports several autoloading strategies to help PHP applications load classes and files automatically. The main strategies are PSR-4 , classmap , and files . Each serves a different purpose and is suited for different use cases.

PSR-4 – Namespace-based Class Autoloading
PSR-4 is the most commonly used autoloading standard in modern PHP projects. It maps namespaces to directories, allowing Composer to locate and load classes based on their fully qualified class name.
How it works:

- Namespace prefixes are mapped to specific directories.
- Class names are converted to file paths by replacing namespace separators (
\
) with directory separators and appending.php
. - The class name must match the file name exactly (case-sensitive).
Example in composer.json
:
{ "autoload": { "psr-4": { "App\\": "src/", "Tests\\": "tests/" } } }
With this configuration:

- The class
App\Controller\HomeController
is expected to be insrc/Controller/HomeController.php
. - The class
Tests\Unit\ExampleTest
goes intests/Unit/ExampleTest.php
.
Key points:
- No need to list individual files.
- Supports subdirectories automatically.
- Ideal for applications and packages using proper namespaces.
classmap – Generated Class Map
The classmap strategy tells Composer to scan specified directories and generate a full list (map) of all classes found, regardless of naming conventions.
How it works:
- Composer parses every PHP file in the given directories.
- It builds a map of class names to file paths.
- This map is stored in
vendor/composer/autoload_classmap.php
.
Example in composer.json
:
{ "autoload": { "classmap": [ "legacy/", "src/" ] } }
Use cases:
- Loading classes that don't follow PSR-4 naming (eg, legacy code).
- Files with multiple classes or unconventional names.
- When you need predictable autoloading behavior without relying on naming standards.
Note: You must run composer dump-autoload
whenever you add or rename classes in classmap directories.
files – Explicit File Inclusion
The files strategy lets you specify individual PHP files to be included once on every request, regardless of whether their classes or functions are used.
How it works:
- Composer includes these files unconditionally when the autoloader is initialized.
- Useful for files that define global functions, constants, or side-effect code.
Example in composer.json
:
{ "autoload": { "files": [ "src/helpers.php", "src/constants.php" ] } }
Use cases:
- Global helper functions (eg,
function_str_contains()
). - Defining constants or configuration logic.
- Bootstrapping code that needs to run on every request.
Caution: Overusing this can slow down your app, as these files are loaded even if not needed.
Summary
Strategy | Best For | Autodiscovery | Manual File List |
---|---|---|---|
PSR-4 | Modern namespaced classes | Yes | No |
classmap | Legacy or non-standard classes | Yes (via scan) | No (but rescan needed) |
files | Global functions/constants | No | Yes (explicit paths) |
You can combine all three in the same composer.json
. After changes, run composer dump-autoload
to regenerate the autoload files.
Basically, use PSR-4 for new code, classmap for old or irregular code, and files for functions and setup logic.
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