View file vendor/slim/slim/Slim/Middleware/ContentLengthMiddleware.php

<?php

/**
 * Slim Framework (https://slimframework.com)
 *
 * @license https://github.com/slimphp/Slim/blob/4.x/LICENSE.md (MIT License)
 */

declare(strict_types=1);

namespace Slim\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

class ContentLengthMiddleware implements MiddlewareInterface
{
    /**
     * @param ServerRequestInterface  $request
     * @param RequestHandlerInterface $handler
     * @return ResponseInterface
     */
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $response = $handler->handle($request);

        // Add Content-Length header if not already added
        $size = $response->getBody()->getSize();
        if ($size !== null && !$response->hasHeader('Content-Length')) {
            $response = $response->withHeader('Content-Length', (string) $size);
        }

        return $response;
    }
}