PHP provides many different ways to download a page(file) through HTTP, the simplest way is using file_get_contents function which is suitable for relatively small files. If try to download large file with file_get_contents the PHP allowed memory (configured by memory_limit directive) may be exhausted with a fatal error. What’s why we need a portable way to deal with large file, where fsockopen comes in.
There are two main conditions we should consider: Content-Lenght specified or Chunked data.
0. Initiate socket connection
Use fsockopen() to initiate the socket connection. You’d better specify the error number, error message and timeout parameters, and process the error if exists.
$url = 'http://test.example.com/fetch_file.php?file=testfile.iso';
if (preg_match_all('#http://([^/]+)(/.+)#i', $url, $matches)) {
$host = $matches[1][0];
$path = $matches[2][0];
} else {
die('Invalid URl');
}
$fp = fsockopen($host, 80, $errno, $error, 30);
//Open a file pointer for write
$wfp = fopen('file-write-to', 'w');
//specify the block size to read
$readBlockSize = 512;
1. Content-Length
If the Content-Length is specified by HTTP response header, the reading is straightforward just as reading general files.
Snippet use to read response body:
$data = fread($fp, $readBlockSize);
fwrite($wfp, $data);
2. Chunked
For chuncked encoding, there is a different data format, here is a quotation from WikiPedia:
Each chunk starts with the number of octets of the data it embeds expressed in ASCII followed by optional parameters (chunk extension) and a terminating CRLF sequence, followed by the chunk data. The chunk is terminated by CRLF. If chunk extensions are provided, the chunk size is terminated by a semicolon followed with the extension name and an optional equal sign and value.
The last-chunk is a regular chunk, with the exception that its length is zero.
The encoded data looks like this:
4
Wiki
5
pedia
E
in
chunks.
0
So we have to address the response chunk by chunk. Snippet to do so:
if ($chunk_length === false) {
$data = trim(fgets($fp, 128));
$chunk_length = hexdec($data);
} else if ($chunk_length > 0) {
$read_length = $chunk_length > $readBlockSize ? $readBlockSize : $chunk_length;
$chunk_length -= $read_length;
$data = fread($fp, $read_length);
fwrite($wfp, $data);
if ($chunk_length <= 0) {
fseek($fp, 2, SEEK_CUR);
$chunk_length = false;
}
} else {
break;
}
The full script:
* @copyright (C) 2013 James Tang.
*/
set_time_limit(600);
ignore_user_abort(true);
//$url = 'http://test.example.com/fetch_file.php?file=testfile.iso';
//$saveToFile = 'tmp.iso';
$url = 'http://test.example.com/fetch_file.php?file=tmp.gz';
$saveToFile = 'tmp.gz';
if (preg_match_all('#http://([^/]+)(/.+)#i', $url, $matches)) {
$host = $matches[1][0];
$path = $matches[2][0];
} else {
die('Invalid URl');
}
$fp = fsockopen($host, 80, $errno, $error, 30);
$readBlockSize = 512;
if ($fp) {
$wfp = fopen($saveToFile, 'w');
if ($wfp) {
$request = "GET $path HTTP/1.1\r\n";
$request .= "Host: $host\r\n";
$request .= "Connection: close\r\n";
$request .= "User-Agent: php-download/1.0\r\n";
$request .= "\r\n";
fwrite($fp, $request);
$body_start = false;
$md5sum = '';
$content_length = false;
$chunk_length = false;
$startLine = fgets($fp, 128);
if ($startLine && preg_match('#^HTTP/1.\d?\s+200\s+#', $startLine)) {
while (!feof($fp)) {
if (!$body_start) {
$header = fgets($fp, 128);
echo $header;
$colon_pos = strpos($header, ':');
$header_name = strtolower(trim(substr($header, 0, $colon_pos)));
$header_value = trim(substr($header, $colon_pos+1));
if ($header_name == 'content-md5') {
$md5sum = bin2hex(base64_decode($header_value));
} else if ($header_name == 'content-length') {
$content_length = (int) $header_value;
}
if ($header == "\r\n") {
$body_start = true;
echo "Reading data...\n";
}
} else {
if ($content_length !== false && $content_length > 0) {
$data = fread($fp, $readBlockSize);
fwrite($wfp, $data);
} else {
if ($chunk_length === false) {
$data = trim(fgets($fp, 128));
$chunk_length = hexdec($data);
} else if ($chunk_length > 0) {
$read_length = $chunk_length > $readBlockSize ? $readBlockSize : $chunk_length;
$chunk_length -= $read_length;
$data = fread($fp, $read_length);
fwrite($wfp, $data);
if ($chunk_length <= 0) {
fseek($fp, 2, SEEK_CUR);
$chunk_length = false;
}
} else {
break;
}
}
}
}
} else {
echo "Failed to read data: " . $startLine . "\n";
}
fclose($wfp);
if ($md5sum && strlen($md5sum) > 0) {
$md5sum_check = bin2hex(md5_file($saveToFile, true));
if ($md5sum_check != $md5sum) {
echo 'MD5 checksum does not match: ' . $md5sum_check . "\n";
} else {
echo "MD5 checksum match\n";
}
} else {
echo "No MD5 checksum detected\n";
}
//unlink($saveToFile);
}
fclose($fp);
} else {
echo 'Error: ' . $errno . '#' . $error . "
\n";
}
3. Problems
The $readBlockSize value is critical, if too large it may cause problem. When I test on remote server with $readBlockSize=4096, the downloaded file was not identical to source file. This problem must be caused by transfer rate: when you try to read 4096 bytes from the response body, but if less than 4096 bytes was prepared, then the reading sequence is disrupted. At last I found 512 works fine for me.
4. Reference
1. http://en.wikipedia.org/wiki/Chunked_transfer_encoding
2. http://tools.ietf.org/html/rfc2616#page-118
