composer/composer · critical · RuntimeException

Composer 2.3.0 dropped support for autoloading on PHP <5.6…

Error message

Composer 2.3.0 dropped support for autoloading on PHP <5.6 and you are running {php_version}, please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.

What it means

This error is thrown by the generated vendor/autoload.php file when it is loaded on a PHP version older than 5.6. Composer 2.3.0 removed support for autoloading on PHP < 5.6, so the generator (AutoloadGenerator.php) embeds a PHP_VERSION_ID guard at the top of every generated autoload.php. When the guard trips it sends a 500 HTTP status header (for web requests), prints the error to STDERR (for CLI), and throws a RuntimeException to halt execution.

Solutions

1. Upgrade PHP on the server running the application to 5.6 or later (7.2+ recommended for Composer 2.3).
2. If you cannot upgrade PHP, downgrade Composer to the 2.2 LTS line: run 'composer self-update --2.2' and reinstall dependencies.
3. If using CI, update the PHP version in your pipeline configuration (e.g. .github/workflows, .circleci/config.yml).
4. Verify the PHP version with 'php -v' on the exact environment where autoload.php is loaded (CLI vs web SAPI may differ).

Example fix

# before: running on PHP 5.5
require __DIR__.'/vendor/autoload.php';
# throws RuntimeException

# after: pin Composer to 2.2 LTS
#   composer self-update --2.2
# then reinstall:
#   composer install
require __DIR__.'/vendor/autoload.php';

// Before loading autoload.php, verify the PHP version is compatible
if (PHP_VERSION_ID < 50600) {
    // Do not load autoload.php; fall back to Composer 2.2 or upgrade PHP
    fwrite(STDERR, 'PHP ' . PHP_VERSION . ' is too old for this autoload. Need >= 5.6.');
    exit(1);
}
require __DIR__ . '/vendor/autoload.php';

// Check whether the running PHP satisfies the Composer 2.3 autoloader guard
function phpSupportsComposer23Autoload(): bool {
    return PHP_VERSION_ID >= 50600;
}

try {
    require __DIR__ . '/vendor/autoload.php';
} catch (\RuntimeException $e) {
    if (str_contains($e->getMessage(), 'dropped support for autoloading on PHP')) {
        // Handle: upgrade PHP or regenerate autoload with Composer 2.2 LTS
        fwrite(STDERR, $e->getMessage());
        exit(1);
    }
    throw $e;
}

When it happens

Trigger: Loading vendor/autoload.php on PHP 5.5 or earlier after dependencies were installed with Composer 2.3+. The guard at AutoloadGenerator.php:984 checks PHP_VERSION_ID < 50600 and throws at line 996. This is generated code injected into the AUTOLOAD template string, not code that runs during normal Composer operation.

Common situations: Deploying to a production server or CI runner that has an older PHP version than the development machine. Upgrading Composer to 2.3+ on a project still hosted on PHP 5.5. Shared hosting environments where the CLI PHP and web PHP versions differ.

Related errors

• Composer detected issues in your platform
• A non-empty PSR-4 prefix must end with a namespace…
• At least the plugin package should always be autoloaded for…
• Can't run git rev-list. You must ensure to run compile from…
• Failed getting info from curl handle

AI-assisted analysis of composer/composer@c435d285c9 (2026-08-07). Data as JSON: /api/errors/be6b142abd90a9b3. Report an issue: GitHub.

Appendix: source

Thrown at src/Composer/Autoload/AutoloadGenerator.php:996

        return <<<AUTOLOAD
<?php

// autoload.php @generated by Composer

if (PHP_VERSION_ID < 50600) {
    if (!headers_sent()) {
        header('HTTP/1.1 500 Internal Server Error');
    }
    \$err = 'Composer 2.3.0 dropped support for autoloading on PHP <5.6 and you are running '.PHP_VERSION.', please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.'.PHP_EOL;
    if (!ini_get('display_errors')) {
        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
            fwrite(STDERR, \$err);
        } elseif (!headers_sent()) {
            echo \$err;
        }
    }
    throw new RuntimeException(\$err);
}

require_once $vendorPathToTargetDirCode;

return ComposerAutoloaderInit$suffix::getLoader();

AUTOLOAD;
    }

    /**
     * @param string $vendorPathCode unused in this method
     * @param string $appBaseDirCode unused in this method
     * @param string $prependAutoloader 'true'|'false'
     * @return string
     */
    protected function getAutoloadRealFile(bool $useClassMap, bool $useIncludePath, ?string $targetDirLoader, bool $useIncludeFiles, string $vendorPathCode, string $appBaseDirCode, string $suffix, bool $useGlobalIncludePath, string $prependAutoloader, bool $checkPlatform)
    {
        $file = <<<HEADER

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