强制文件下载处理

Question

I am trying to understand how the response is created for force downloads and how the browser handles it.

Following this article here: tutorial .

I have a script that sends a file as response to download.

<?php
// it's a zip file
header('Content-Type: application/zip');
// 1 million bytes (about 1megabyte)
header('Content-Length: 1000000');
// load a download dialogue, and save it as download.zip
header('Content-Disposition: attachment; filename="download.zip"');

// 1000 times 1000 bytes of data
for ($i = 0; $i < 1000; $i++) {
    echo str_repeat(".",1000);

    // sleep to slow down the download
    // sleep(5);
}
sleep(5);

When the sleep() function is inside the loop, it waits for sometime before the file starts downloading.

But when placed outside the loop, file starts downloading immediately.

Can anyone please help me understand this behaviour?

Answer 1

The problem in the second case is you send the file to the client before you call the sleep function. You can store the output in an internal buffer and send it after the sleep function. (I don't recommend this for production use.) Try this modified program:

<?php
// it's a zip file
header('Content-Type: application/zip');
// 1 million bytes (about 1megabyte)
header('Content-Length: 1000000');
// load a download dialogue, and save it as download.zip
header('Content-Disposition: attachment; filename="download.zip"');

//Turn on output buffering
ob_start();

// 1000 times 1000 bytes of data
for ($i = 0; $i < 1000; $i++) {
    echo str_repeat(".",1000);

    // sleep to slow down the download
    // sleep(5);
}

//Store the contents of the output buffer
$buffer = ob_get_contents();
// Clean the output buffer and turn off output buffering
ob_end_clean();

sleep(5);

echo $buffer;