Description:
------------
When the following code is run in a browser on a very large file php will first read in the entire file into memory and only afterwards will it display a download dialog in the browser. This is even though I'm reading in 8192 bytes at a time. 

On a large file this results in a delay of several seconds.

Using readfile() doesn't have this problem but it is unacceptable since I will need to add code to throttle download speed.

Reproduce code:
---------------
<?php
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=largeFile");
header("Content-Length: " . filesize("largeFile")); 

$handle = fopen ("largeFile", "rb");
do {
    $data = fread($handle, 8192);
    if (strlen($data) == 0) {
        break;
    }
    echo $data;
} while(true);
fclose ($handle);
?>


Expected result:
----------------
PHP should first show the download dialog and then start reading the file. This is what happens with readfile()

Actual result:
--------------
The file will first get read entirely into memory resulting in a large delay. Only then is the user prompted with a download dialog.