由于我们不希望公开可下载文件的绝对路径，因此我们正在使用PHP脚本来隧穿文件下载。

header("Content-Type: $ctype");
header("Content-Length: " . filesize($file));
header("Content-Disposition: attachment; filename=\"$fileName\"");
readfile($file);

不幸的是，我们注意到最终用户无法恢复通过此脚本进行的下载。

有什么方法可以通过这种基于PHP的解决方案来支持可恢复的下载吗？

解决方案

是的。支持字节范围。参见RFC 2616第14.35节。

基本上，这意味着我们应该阅读Range标头，并从指定的偏移量开始提供文件。

这意味着我们不能使用readfile()，因为它可以提供整个文件。相反，请先使用fopen()，然后使用fseek()到正确的位置，然后使用fpassthru()服务该文件。

通过Range标头完成HTTP中的下载。如果请求中包含" Range"标头，并且其他指示符(例如，" If-Match"，" If-Unmodified-Since")指示自下载开始以来内容未更改，则给出206响应代码(而不是200)，请在" Content-Range"标头中指明要返回的字节范围，然后在响应正文中提供该范围。

我不知道如何在PHP中做到这一点。

是的，我们可以为此使用Range标头。我们需要再给客户端提供3个标头以进行完整下载：

header ("Accept-Ranges: bytes");
header ("Content-Length: " . $fileSize);
header ("Content-Range: bytes 0-" . $fileSize - 1 . "/" . $fileSize . ";");

与下载中断相比，我们需要通过以下方法检查"范围请求"标头：

$headers = getAllHeaders ();
$range = substr ($headers['Range'], '6');

在这种情况下，请不要忘记为内容提供206状态代码：

header ("HTTP/1.1 206 Partial content");
header ("Accept-Ranges: bytes");
header ("Content-Length: " . $remaining_length);
header ("Content-Range: bytes " . $start . "-" . $to . "/" . $fileSize . ";");

我们将从请求标头中获取$ start和$ to变量，并使用fseek()来查找文件中的正确位置。

我们需要做的第一件事是在所有响应中发送Accept-Ranges：bytes标头，以告诉客户端我们支持部分内容。然后，如果收到带有" Range：bytes = xy`？"标头的请求(" x"和" y"为数字)，则我们解析客户端请求的范围，照常打开文件，在前面查找" x"个字节并发送下一个y个x字节。还将响应设置为" HTTP / 1.0 206部分内容"。

如果没有进行任何测试，这或者多或者少会起作用：

$filesize = filesize($file);

$offset = 0;
$length = $filesize;

if ( isset($_SERVER['HTTP_RANGE']) ) {
    // if the HTTP_RANGE header is set we're dealing with partial content

    $partialContent = true;

    // find the requested range
    // this might be too simplistic, apparently the client can request
    // multiple ranges, which can become pretty complex, so ignore it for now
    preg_match('/bytes=(\d+)-(\d+)?/', $_SERVER['HTTP_RANGE'], $matches);

    $offset = intval($matches[1]);
    $length = intval($matches[2]) - $offset;
} else {
    $partialContent = false;
}

$file = fopen($file, 'r');

// seek to the requested offset, this is 0 if it's not a partial content request
fseek($file, $offset);

$data = fread($file, $length);

fclose($file);

if ( $partialContent ) {
    // output the right headers for partial content

    header('HTTP/1.1 206 Partial Content');

    header('Content-Range: bytes ' . $offset . '-' . ($offset + $length) . '/' . $filesize);
}

// output the regular HTTP headers
header('Content-Type: ' . $ctype);
header('Content-Length: ' . $filesize);
header('Content-Disposition: attachment; filename="' . $fileName . '"');
header('Accept-Ranges: bytes');

// don't forget to send the data too
print($data);

我可能错过了一些显而易见的事情，并且我绝对可以忽略一些潜在的错误来源，但这应该是一个开始。

这里有部分内容的描述，我在文档页面上找到了一些有关部分内容的信息以供阅读。

解决此问题的一种非常不错的方法，而无需"滚动自己的" PHP代码，是使用mod_xsendfile Apache模块。然后在PHP中，只需设置适当的标头即可。 Apache开始做自己的事情。

header("X-Sendfile: /path/to/file");
header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; file=\"filename\"");

编辑2017/01我写了一个库在PHP> = 7.0 https://github.com/DaveRandom/Resume中做到这一点

EDIT 2016/02代码完全重写为一组模块化工具，仅作为示例用法，而不是单片函数。下面的评论中提到的更正已被并入。

经过测试的有效解决方案(很大程度上取决于Theo的上述答案)，它通过一组独立工具来处理可恢复的下载。此代码需要PHP 5.4或者更高版本。

该解决方案仍然只能满足每个请求一个范围，但是在我可以想到的任何情况下，如果使用标准浏览器，这都不会造成问题。

<?php

/**
 * Get the value of a header in the current request context
 *
 * @param string $name Name of the header
 * @return string|null Returns null when the header was not sent or cannot be retrieved
 */
function get_request_header($name)
{
    $name = strtoupper($name);

    // IIS/Some Apache versions and configurations
    if (isset($_SERVER['HTTP_' . $name])) {
        return trim($_SERVER['HTTP_' . $name]);
    }

    // Various other SAPIs
    foreach (apache_request_headers() as $header_name => $value) {
        if (strtoupper($header_name) === $name) {
            return trim($value);
        }
    }

    return null;
}

class NonExistentFileException extends \RuntimeException {}
class UnreadableFileException extends \RuntimeException {}
class UnsatisfiableRangeException extends \RuntimeException {}
class InvalidRangeHeaderException extends \RuntimeException {}

class RangeHeader
{
    /**
     * The first byte in the file to send (0-indexed), a null value indicates the last
     * $end bytes
     *
     * @var int|null
     */
    private $firstByte;

    /**
     * The last byte in the file to send (0-indexed), a null value indicates $start to
     * EOF
     *
     * @var int|null
     */
    private $lastByte;

    /**
     * Create a new instance from a Range header string
     *
     * @param string $header
     * @return RangeHeader
     */
    public static function createFromHeaderString($header)
    {
        if ($header === null) {
            return null;
        }

        if (!preg_match('/^\s*(\S+)\s*(\d*)\s*-\s*(\d*)\s*(?:,|$)/', $header, $info)) {
            throw new InvalidRangeHeaderException('Invalid header format');
        } else if (strtolower($info[1]) !== 'bytes') {
            throw new InvalidRangeHeaderException('Unknown range unit: ' . $info[1]);
        }

        return new self(
            $info[2] === '' ? null : $info[2],
            $info[3] === '' ? null : $info[3]
        );
    }

    /**
     * @param int|null $firstByte
     * @param int|null $lastByte
     * @throws InvalidRangeHeaderException
     */
    public function __construct($firstByte, $lastByte)
    {
        $this->firstByte = $firstByte === null ? $firstByte : (int)$firstByte;
        $this->lastByte = $lastByte === null ? $lastByte : (int)$lastByte;

        if ($this->firstByte === null && $this->lastByte === null) {
            throw new InvalidRangeHeaderException(
                'Both start and end position specifiers empty'
            );
        } else if ($this->firstByte < 0 || $this->lastByte < 0) {
            throw new InvalidRangeHeaderException(
                'Position specifiers cannot be negative'
            );
        } else if ($this->lastByte !== null && $this->lastByte < $this->firstByte) {
            throw new InvalidRangeHeaderException(
                'Last byte cannot be less than first byte'
            );
        }
    }

    /**
     * Get the start position when this range is applied to a file of the specified size
     *
     * @param int $fileSize
     * @return int
     * @throws UnsatisfiableRangeException
     */
    public function getStartPosition($fileSize)
    {
        $size = (int)$fileSize;

        if ($this->firstByte === null) {
            return ($size - 1) - $this->lastByte;
        }

        if ($size <= $this->firstByte) {
            throw new UnsatisfiableRangeException(
                'Start position is after the end of the file'
            );
        }

        return $this->firstByte;
    }

    /**
     * Get the end position when this range is applied to a file of the specified size
     *
     * @param int $fileSize
     * @return int
     * @throws UnsatisfiableRangeException
     */
    public function getEndPosition($fileSize)
    {
        $size = (int)$fileSize;

        if ($this->lastByte === null) {
            return $size - 1;
        }

        if ($size <= $this->lastByte) {
            throw new UnsatisfiableRangeException(
                'End position is after the end of the file'
            );
        }

        return $this->lastByte;
    }

    /**
     * Get the length when this range is applied to a file of the specified size
     *
     * @param int $fileSize
     * @return int
     * @throws UnsatisfiableRangeException
     */
    public function getLength($fileSize)
    {
        $size = (int)$fileSize;

        return $this->getEndPosition($size) - $this->getStartPosition($size) + 1;
    }

    /**
     * Get a Content-Range header corresponding to this Range and the specified file
     * size
     *
     * @param int $fileSize
     * @return string
     */
    public function getContentRangeHeader($fileSize)
    {
        return 'bytes ' . $this->getStartPosition($fileSize) . '-'
             . $this->getEndPosition($fileSize) . '/' . $fileSize;
    }
}

class PartialFileServlet
{
    /**
     * The range header on which the data transmission will be based
     *
     * @var RangeHeader|null
     */
    private $range;

    /**
     * @param RangeHeader $range Range header on which the transmission will be based
     */
    public function __construct(RangeHeader $range = null)
    {
        $this->range = $range;
    }

    /**
     * Send part of the data in a seekable stream resource to the output buffer
     *
     * @param resource $fp Stream resource to read data from
     * @param int $start Position in the stream to start reading
     * @param int $length Number of bytes to read
     * @param int $chunkSize Maximum bytes to read from the file in a single operation
     */
    private function sendDataRange($fp, $start, $length, $chunkSize = 8192)
    {
        if ($start > 0) {
            fseek($fp, $start, SEEK_SET);
        }

        while ($length) {
            $read = ($length > $chunkSize) ? $chunkSize : $length;
            $length -= $read;
            echo fread($fp, $read);
        }
    }

    /**
     * Send the headers that are included regardless of whether a range was requested
     *
     * @param string $fileName
     * @param int $contentLength
     * @param string $contentType
     */
    private function sendDownloadHeaders($fileName, $contentLength, $contentType)
    {
        header('Content-Type: ' . $contentType);
        header('Content-Length: ' . $contentLength);
        header('Content-Disposition: attachment; filename="' . $fileName . '"');
        header('Accept-Ranges: bytes');
    }

    /**
     * Send data from a file based on the current Range header
     *
     * @param string $path Local file system path to serve
     * @param string $contentType MIME type of the data stream
     */
    public function sendFile($path, $contentType = 'application/octet-stream')
    {
        // Make sure the file exists and is a file, otherwise we are wasting our time
        $localPath = realpath($path);
        if ($localPath === false || !is_file($localPath)) {
            throw new NonExistentFileException(
                $path . ' does not exist or is not a file'
            );
        }

        // Make sure we can open the file for reading
        if (!$fp = fopen($localPath, 'r')) {
            throw new UnreadableFileException(
                'Failed to open ' . $localPath . ' for reading'
            );
        }

        $fileSize = filesize($localPath);

        if ($this->range == null) {
            // No range requested, just send the whole file
            header('HTTP/1.1 200 OK');
            $this->sendDownloadHeaders(basename($localPath), $fileSize, $contentType);

            fpassthru($fp);
        } else {
            // Send the request range
            header('HTTP/1.1 206 Partial Content');
            header('Content-Range: ' . $this->range->getContentRangeHeader($fileSize));
            $this->sendDownloadHeaders(
                basename($localPath),
                $this->range->getLength($fileSize),
                $contentType
            );

            $this->sendDataRange(
                $fp,
                $this->range->getStartPosition($fileSize),
                $this->range->getLength($fileSize)
            );
        }

        fclose($fp);
    }
}

用法示例：

<?php

$path = '/local/path/to/file.ext';
$contentType = 'application/octet-stream';

// Avoid sending unexpected errors to the client - we should be serving a file,
// we don't want to corrupt the data we send
ini_set('display_errors', '0');

try {
    $rangeHeader = RangeHeader::createFromHeaderString(get_request_header('Range'));
    (new PartialFileServlet($rangeHeader))->sendFile($path, $contentType);
} catch (InvalidRangeHeaderException $e) {
    header("HTTP/1.1 400 Bad Request");
} catch (UnsatisfiableRangeException $e) {
    header("HTTP/1.1 416 Range Not Satisfiable");
} catch (NonExistentFileException $e) {
    header("HTTP/1.1 404 Not Found");
} catch (UnreadableFileException $e) {
    header("HTTP/1.1 500 Internal Server Error");
}

// It's usually a good idea to explicitly exit after sending a file to avoid sending any
// extra data on the end that might corrupt the file
exit;