文件下载过程中被暂停或中断,恢复下载时如果重新下载整个文件,会消耗额外的网络流量及时间.断点续传,可以按照Client的需求,对文件进行分段下载,返回Client需要的文件内容,节省流量与时间.断点续传在 HTTP/1.1 (RFC2616)中得到支持.HTTP/1.0不支持断点续传.

当然，使用PHP做下载并不太好，这里仅做为讨论。

下面以resumedownload.txt纯文本为例,内容如下:

1234567890

1.Client:

在请求的http header中添加 Range: bytes=3-9表明需要获取文件的哪一个片段,值得注意的是,Range bytes是从0开始计数的,也就是说Range: bytes=0-0 表示请求第一个byte,即”1″

2.Server:

在响应的http header中添加HTTP/1.1 206 Partial Content,表明输出的为文件片段(partial就是部分,局部的意思!),

在响应的http header中添加Content-Range: bytes 0-9/10,表明输出的是哪一片段的文件.(其中0表示起始位置,9表示终止位置,最后面的10表示整个文件的大小)

下面是php实现(Github ResumeDownload):

<?php

/**
 * A download resume class
 * 
 * Client request header example:
 * Range: bytes=0-199
 * 
 * Sever usage:
 * $url = 'http://www.xxx.com/xxx.zip';
 * $rd = new ResumeDownload($url);
 * $rd->download();
 *
 * @author aiddroid
 * @date 2013-11-22
 * @encoding UTF-8
 */
class ResumeDownload {

    //file path
    private $filePath;
    //file name
    private $fileName;
    //file handler
    private $fileHandler;
    //the start of download
    private $rangeStart;
    //the end of download
    private $rangeEnd;
    //file size
    private $fileSize;
    private $log;

    /**
     * construction
     * @param type $filePath file path
     * @return type
     */
    function __construct($filePath, $log = false) {
	$this->log = $log;
	$this->filePath = $filePath;
	//check if file exists and readable
	if (!is_file($this->filePath) || !($this->fileHandler = fopen($this->filePath, 'rb'))) {
	    return $this->sendError();
	}
	$this->fileName = basename($this->filePath);
	$this->fileSize = filesize($this->filePath);

	//get range value from http header
	$this->initDownloadRange();
    }

    /**
     * resume download
     */
    public function download() {
	$this->sendHeader();
	$this->sendContent();
    }

    /*
     * get range start and end from http header
     */

    private function initDownloadRange() {
	preg_match("/^bytes=(\d?)-(\d*?)$/i", $_SERVER['HTTP_RANGE'], $match);
	$this->rangeStart = intval($match[1]);
	$this->rangeEnd = intval($match[2]);
	//log
	$this->log("$this->rangeStart $this->rangeEnd $this->fileSize", __FUNCTION__);

	//check range value
	if ($this->rangeStart < 0 || $this->rangeStart >= $this->fileSize || (!empty($this->rangeEnd) && $this->rangeEnd < $this->rangeStart) || $this->rangeEnd >= $this->fileSize) {
	    return $this->sendError('HTTP/1.1 416 Requested Range Not Satisfiable');
	}
    }

    /*
     * send output header
     */

    private function sendHeader() {
	//clear php errors or warnings
	ob_clean();
	header('Content-Length: ' . ($this->rangeEnd - $this->rangeStart));

	if ($this->rangeStart > 0) {
	    header('HTTP/1.1 206 Partial Content');
	    //Content-Range: bytes 21010-47021/47022
	    header('Content-Range: bytes ' . $this->rangeStart . '-' . $this->rangeEnd . '/' . $this->fileSize);
	} else {
	    header('Accept-Ranges: bytes');
	}

	header('Content-Type: application/octet-stream');
	//for filename
	header('Content-Disposition: attachment;filename=' . $this->fileName);
    }

    /**
     * send real data
     */
    private function sendContent() {
	//seek file point to read
	fseek($this->fileHandler, $this->rangeStart);
	//read file as range
	if (!empty($this->rangeEnd)) {
	    //Range: bytes=0-0 -> read the first byte
	    $content = fread($this->fileHandler, ($this->rangeEnd - $this->rangeStart + 1));
	    echo $content;
	    return;
	}
	//file passthrough?
	fpassthru($this->fileHandler);
    }

    /**
     * send error
     */
    private function sendError($error) {
	header($error ? $error : 'HTTP/1.0 404 Not Found');
    }

    /**
     * log
     * @param type $content
     * @param type $logFile
     */
    private function log($content, $logFile = '') {
	if ($this->log) {
	    $logFile = empty($logFile) ? __FUNCTION__ : $logFile;
	    $logFile = __CLASS__ . '.' . $logFile;
	    error_log(date('Y-m-d H:i:s') . " $content \n", 3, $logFile . '.' . date('Ymd') . '.log');
	}
    }

}