Composer platform configuration and platform-check execution timing

Yes, the platform-check script is executed in all autoloader variants, including those generated with -o (classmap) or --apcu-autoloader. These optimization flags change how Composer resolves class locations, but they do not modify the bootstrap sequence of the autoloader itself.

Execution Mechanism

When the platform configuration is used to simulate a specific environment, Composer generates a platform_check.php file. This file is not part of the class-loading logic; instead, it is a hard-coded requirement within the autoloader's entry point. Because vendor/autoload.php is typically the first file required by an application's entry point (e.g., index.php), the platform validation occurs before any application code or third-party classes are loaded.

Impact of Optimization Modes

Optimization Mode	Effect on Class Loading	Effect on Platform Check
Default	PSR-4/PSR-0 lookup	Executed via autoload.php
-o (Classmap)	Static array lookup	Executed via autoload.php
--apcu-autoloader	Cached lookup in APCu	Executed via autoload.php

Verification Steps

To verify that the check remains active in your specific environment, you can perform the following manual inspections:

1. Check for the file: Confirm that vendor/composer/platform_check.php exists after running your install or update command.
2. Inspect the entry point: Open vendor/autoload.php. You should see a require statement specifically targeting the platform_check.php file.
3. Test optimizations: Run the following commands and re-inspect vendor/autoload.php to ensure the requirement persists:

   composer dump-autoload -o
   composer dump-autoload --apcu

Assumptions and Constraints

This behavior assumes you are using Composer 2.x, where the platform-check feature was standardized. It is important to note that the platform check only validates the environment against the constraints defined in composer.json or the platform config; it does not perform a deep analysis of the actual source code requirements.

Diagnostic Note: Are you using a custom autoloader implementation or a framework that bypasses vendor/autoload.php? If so, the platform check will be skipped entirely.