composer/composer · critical · RuntimeException

Composer 2.3.0 dropped support for autoloading on PHP <5.6 a

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 is generated source code: AutoloadGenerator writes this RuntimeException into vendor/autoload.php, and it is thrown at runtime when that generated autoloader is required on a PHP version below 5.6. Composer 2.3.0+ relies on PHP 5.6+ language features, so the generated autoloader refuses to run on older runtimes and sends a 500 header if in a web context.

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

View on GitHub (pinned to 6ffc117740)

Solutions

1. Upgrade the runtime PHP to 5.6 or newer (recommended).
2. Downgrade Composer to 2.2 LTS on the build machine with `composer self-update --2.2` so the generated autoloader stays 5.x compatible.
3. Align build and production PHP versions so the generated autoloader targets a supported runtime.

Example fix

// before (build with Composer 2.3, deploy to PHP 5.5)
composer install  # generates autoloader requiring PHP >= 5.6

// after
composer self-update --2.2
composer install  # generates a 5.x-compatible autoloader

if (PHP_VERSION_ID < 50600) { throw new \RuntimeException('PHP < 5.6 is unsupported by this autoloader; use Composer 2.2 LTS to generate it.'); }

function autoloadCompatiblePhp(): bool { return PHP_VERSION_ID >= 50600; }

try { require __DIR__ . '/vendor/autoload.php'; } catch (\RuntimeException $e) { http_response_code(500); error_log($e->getMessage()); // fall back to 2.2-generated autoloader or upgrade PHP exit(1); }

When it happens

Trigger: Deploying an application whose dependencies were installed with Composer 2.3+ onto a server running PHP 5.5 or earlier, then including vendor/autoload.php. The throw occurs in the generated autoloader, not in the Composer binary.

Common situations: Legacy hosting with old PHP; CI/build image using a newer PHP than the production runtime; CI installing with Composer 2.3 but deploying to a PHP 5.5 box.

Related errors

• There is no version of Composer available for your PHP versi
• Cannot use table format with {io_class}
• Invalid format "{format}".
• Not enough arguments (missing: "packages").
• Composer update failed: the "{localFilename}" is not accessi

AI-assisted analysis of composer/composer@6ffc117740 (2026-08-07). Data as JSON: /api/errors/be6b142abd90a9b3. Report an issue: GitHub.