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    <h3>Editing: PrepareBodyMiddleware.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

namespace GuzzleHttp;

use GuzzleHttp\Promise\PromiseInterface;
use Psr\Http\Message\RequestInterface;

/**
 * Prepares requests that contain a body, adding the Content-Length,
 * Content-Type, and Expect headers.
 *
 * @final
 */
class PrepareBodyMiddleware
{
    /**
     * @var callable(RequestInterface, array): PromiseInterface
     */
    private $nextHandler;

    /**
     * @param callable(RequestInterface, array): PromiseInterface $nextHandler Next handler to invoke.
     */
    public function __construct(callable $nextHandler)
    {
        $this-&gt;nextHandler = $nextHandler;
    }

    public function __invoke(RequestInterface $request, array $options): PromiseInterface
    {
        $fn = $this-&gt;nextHandler;

        // Don't do anything if the request has no body.
        if ($request-&gt;getBody()-&gt;getSize() === 0) {
            return $fn($request, $options);
        }

        $modify = [];

        // Add a default content-type if possible.
        if (!$request-&gt;hasHeader('Content-Type')) {
            if ($uri = $request-&gt;getBody()-&gt;getMetadata('uri')) {
                if (is_string($uri) &amp;&amp; $type = Psr7\MimeType::fromFilename($uri)) {
                    $modify['set_headers']['Content-Type'] = $type;
                }
            }
        }

        // Add a default content-length or transfer-encoding header.
        if (!$request-&gt;hasHeader('Content-Length')
            &amp;&amp; !$request-&gt;hasHeader('Transfer-Encoding')
        ) {
            $size = $request-&gt;getBody()-&gt;getSize();
            if ($size !== null) {
                $modify['set_headers']['Content-Length'] = $size;
            } else {
                $modify['set_headers']['Transfer-Encoding'] = 'chunked';
            }
        }

        // Add the expect header if needed.
        $this-&gt;addExpectHeader($request, $options, $modify);

        return $fn(Psr7\Utils::modifyRequest($request, $modify), $options);
    }

    /**
     * Add expect header
     */
    private function addExpectHeader(RequestInterface $request, array $options, array &amp;$modify): void
    {
        // Determine if the Expect header should be used
        if ($request-&gt;hasHeader('Expect')) {
            return;
        }

        $expect = $options['expect'] ?? null;

        // Return if disabled or if you're not using HTTP/1.1 or HTTP/2.0
        if ($expect === false || $request-&gt;getProtocolVersion() &lt; 1.1) {
            return;
        }

        // The expect header is unconditionally enabled
        if ($expect === true) {
            $modify['set_headers']['Expect'] = '100-Continue';
            return;
        }

        // By default, send the expect header when the payload is &gt; 1mb
        if ($expect === null) {
            $expect = 1048576;
        }

        // Always add if the body cannot be rewound, the size cannot be
        // determined, or the size is greater than the cutoff threshold
        $body = $request-&gt;getBody();
        $size = $body-&gt;getSize();

        if ($size === null || $size &gt;= (int) $expect || !$body-&gt;isSeekable()) {
            $modify['set_headers']['Expect'] = '100-Continue';
        }
    }
}
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